Archive for the ‘Technology’ Category

« Older Entries

How Runtime is Calculated for a UPS

Thursday, September 17th, 2009

I’ve mentioned in other posts that runtime is battery. On our websites we use calculated values to determine the runtime based on a particular load. The purpose of this post is to give you an understanding of this process.

The first thing you need to know is that battery discharge is non-linear. For those of you who don’t understand the expression I’ll elaborate. A linear expression is one where, for example, you put two in, you get four out. So it follows that if you put three in, you get six out, or you put five in, you get ten out. EG. whatever you put in  you get twice out. In the non-linear world this doesn’t hold true, for example, you put two in, you get four out, but when you put three in, you get nine out etc.. This non-linearity makes the discharge characteristics very difficult to express mathematically.

Luckily, the battery manufacturers provide us with discharge tables that we can look up, but first we need to know some information about the UPS, the load and about the method.

End of Discharge Point

At what point will the UPS switch off? Your normal 12V lead acid battery contains 6 cells each of a nominal voltage of 2V (so you get 6×2V=12V). In practice the voltage is slightly higher than this and reduces as the battery is discharged. It is important not to allow the battery to become too discharged, so the UPS will monitor the cell voltage and cut off when it gets to a predetermined point. This is usually around 1.7V per cell or 10.2V for 12V battery.

UPS Efficiency?

Well, more precisely, the inverter efficiency. The inverter is used to convert the battery DC power into AC power. There will be losses associated with this. The better the inverter, the lower these losses are. If you’re unsure, use a worst case of say, 80% efficient. This means that for every 100W provided to the load, the batteries will need to provide 125W (simply 100/efficiency).

The Load Power Factor

Is the load purely resistive, or does it have a power factor? We’re only interested in the amount of WATTS that are needed.

Amps or Watts Method?

Firstly, there are two methods for calculating runtime, the Amps method, or the Watts per Cell Method. Generally, Watts per Cell is used for short term discharges and Amps is used for long term discharges.

Process

It’s easier to do this with an example, so let us take a standard server type load with a number of ancillary devices. We know from measurement  that the Ampere draw is 7Amps and we have mains voltage of 235V. Our Watts therefore (assuming unity power factor) is 1645W.

Our UPS has an inverter that is rated at 90% efficient, so the amount of power from the battery required to deliver 1645W is 1645/0.9 = 1828Watts.

Our UPS is a 3KVA, 2.1KW UPS that contains 8 batteries connected in series. Each battery is rated at 12V 7.2 Ah.

 I now need to look up the manufacturers data sheet and I find the following for a 7.2Ah battery:

Constant Current Discharge
F.V. 5MIN 10MIN 15MIN 30MIN 1HR 2HR 3HR 4HR 5HR 8HR 10HR 20HR
1.60V 28.6 18.6 14.3 8.75 5.28 3.06 2.18 1.72 1.42 0.930 0.760 0.400
1.67V 26.8 17.9 13.9 8.69 5.27 3.05 2.17 1.71 1.41 0.930 0.753 0.386
1.70V 26.0 17.6 13.8 8.67 5.26 3.05 2.17 1.71 1.41 0.930 0.750 0.380
1.75V 24.3 16.8 13.3 8.55 5.24 3.03 2.16 1.71 1.41 0.925 0.740 0.370
1.80V 22.5 16.0 12.8 8.42 5.22 3.00 2.15 1.70 1.40 0.920 0.730 0.360
1.85V 20.8 15.2 12.4 8.30 5.20 2.98 2.14 1.70 1.40 0.915 0.720 0.350
Constant Power Discharge
F.V. 5MIN 10MIN 15MIN 30MIN 1HR 2HR 3HR 4HR 5HR 8HR 10HR 20HR
1.60V 57.16 37.16 28.5 17.5 10.56 6.1 4.36 3.45 2.83 1.86 1.51 0.79
1.67V 53.5 35.83 27.83 17.33 10.53 6.06 4.35 3.43 2.81 1.85 1.5 0.77
1.70V 52 35.16 27.5 17.33 10.51 6.05 4.33 3.43 2.81 1.85 1.49 0.76
1.75V 48.5 33.66 26.66 17.16 10.48 6.03 4.31 3.43 2.81 1.83 1.47 0.74
1.80V 45 32 25.66 16.83 10.43 6 4.3 3.41 2.8 1.81 1.45 0.71
1.85V 41.5 30.5 24.83 16.66 10.4 5.98 4.28 3.41 2.8 1.8 1.43 0.69

Constant Power Discharge Method 

Our battery load is 1828W and we have 6×8=48 cells. Therefore our Watts per Cell is 38Wpc.

We know the FV (Final Value or End Of Discharge Point) is 1.7Vpc so looking along the Constant Power Discharge Table, we can see that 52Wpc would give 5 minutes, so we will get more than 5 minutes runtime. We can see that 35.16Wpc would give 10 minutes runtime, so we will get less than this.

So the calculated runtime for this example, based on constant power discharge is between 5 and 10 minutes.

 Constant Current Discharge Method

We have a total of 8×12V batteries in series, giving us a string voltage of 96V. We need to deliver 1828W so our Amperage is 1828/96 (from Power=VoltsxAmps, so Amps=Power/Volts). = 19Amps.

Now looking along the table above in the Constant Current Section, with our FV of 1.7, we see that a 26A discharge would give us 5 minutes, so we will get longer than this. A 17.6Amp discharge would give 10 minutes, so we will get less than this.

Therefore using the constant current discharge method we will get between 5 and 10 minutes runtime.

Working it out in reverse – I want 2 hours runtime – how many batteries do I need?

Using Watts Per Cell Method.

From the table under Constant Power Discharge, we can see that for our FV of 1.7, for a 2 hour runtime we need to have a WPC discharge of no more than 6.05. Our load is 1828W, so we need 1828/6.05 = 302 cells, which is 50.35 batteries. The battery requires a 96V string voltage, based on banks of 8, so we will require 6 banks to get close (that is 48 batteries), or 7 banks to be sure (that is 56 batteries).

Using Constant Current Method.

From the table above under Constant Current Discharge, you can see for our FV of 1.7V we need to have no more than a 3.05A discharge from each of our batteries to achieve a 2 hour runtime.

Our total current draw is 1828/96 = 19Amps (1828Watts load/Battery String Voltage = 96V)

Dividing the 19Amps total current by 3.05 gives us the number of strings needed to achieve 2 hours runtime which is 6.24. Obviously we cannot add in a quarter of a string so we need to round up. In this case we require 7 battery strings, or a total of 56 batteries to achieve a runtime of 2 hours.

Alternatively, you could of course opt for higher capacity batteries, and maintain the same number of batteries. The examples above were using 7.2Ah lead acid batteries but there are other choices available.

Tags: , ,
Posted in Help Guides, Technology, batteries | No Comments »

Power Cuts Damage Hard Drives

Tuesday, September 15th, 2009

Everybody is aware that a sudden loss of power to your PC will result in it switching off and you losing all data that you have accumulated since your last save. Even if the power cut is momentary, this is enough to cause a reboot.

This is bad enough for many people, especially when used for work and you’ve just lost all the data on that report you’d been writing. But did you know that mains fluctuations can cause damage to your hard drive?

A common effect is to hear a repetitious clicking noise coming from your hard drive – the click of death (like the blue screen of death but more fatal if there’s such an analogy). This renders your hard disk inoperable. What does this mean?

Well, your PC cannot function. You cannot access any data that you have on it without specialist services, and you need to replace your Hard Disk Drive (HDD), it cannot be repaired.

The cost of a new hard drive is relatively cheap these days. Probably around the £30 for a no-frills device. But then you have to fit it, and reload all your software. Assuming you’ve got all the CDs/DVD’s, and your access codes for downloaded software, plus you backed up all your important memories from your digital camera (you did didn’t you?), then this should be a breeze. Only taking a full day or so. It’s no laughing matter loading a PC from scratch. You get used to all your settings, software, and although sometimes its good to have a purge you’ll be surprised how long this process can take.

If you needed to recover data from your hard drive, well that’s a different story. You will be charged anywhere upwards of several hundred pounds to have data recovered – if it can be recovered.

To avoid these issues you need to invest in an Uninterruptible Power Supply. These will allow you to work through brief power cuts and shut your computer down (after saving your work), if the outage is longer than a few minutes. Some people provide a degree of power protection for their PC’s by using surge suppressors. Note that these will NOT protect damage to your HDD because of power fluctuations. A surge strip can do nothing to protect against mains voltage variations and power cuts.

How much is an Uninterruptible Power Supply? It depends upon the level of protection you require and the amount of runtime you need, but a basic standard system can be bought for under £30 (plus VAT). Power Inspired’s VIX series Uninterruptible Power Supply fits the bill for a home office / small office PC and is such a low cost it’s a no-brainer to me.

Tags: , , ,
Posted in Applications, Technology | No Comments »

The Benefits of the Modular UPS System

Tuesday, July 21st, 2009

You may have read a lot about the Modular UPS System, and I hope to be able to state some of the key benefits / drawbacks here.

Firstly – expandability. Let us suppose you are developing a data room. The plan is to eventually have, for example, 25 cabinets, each with a power consumption of 3KVA = 75KVA total load. However, at present you only need power for 5 (15KVA), with the remainder being added over the next few years or so.

The sensible approach using the standard Uninterruptible Power Supply would be to fit an 80KVA model. However in the early days it would only be operating at less than 20% capacity. So you’ve shelled out for an 80KVA system that wont be at capacity for a couple of years. For an 80KVA system (excluding battery and installation) you’d be looking at a cost in the region of £8,000, depending on options.

With the Modular UPS, you would fit a 100KVA carrier, and 2x10KVA Power Modules at a cost of around £6,000. You can then add the additional 10KVA power modules as and when required at around £1,500 each.

The benefit here is that the initial outlay is lower, however the total cost will be higher, as you need to add in another 6x 10KVA Power Modules units, making the total cost £15,000 as opposed to £8,000 for the standard Uninterruptible Power Supply.

However, let us now suppose that we want a n+1 redundant solution. So with our standard Uninterruptible Power Supply model, we would put in 2×80KVA UPS Systems, at an upfront cost of £16,000. With the Modular UPS we can put in the 1 extra power module that we need, so our initial upfront cost is 1x 100KVA carrier, and 3x 10KVA Power Modules at a cost of around £7,500.

However, the real benefit is to do with the fact that to achieve n+1 we only need 90KVA of UPS power, as opposed to 160KVA in the configuration above. When the data centre is fully operational we would require 1x 100KVA carrier, and 9x 10KVA Power Modules at a cost of around £16,500. So, slightly more expensive but in an equivalent ball park, however other important factors are that the Modular UPS is in one cabinet with a small footprint, occupying probably half the space of the 2x 80KVA Standard UPS Systems and the fact that the power modules can be easily swapped in the event of a fault – thereby improving on availability figures.

It would be remiss of me however, not to include a third scenario. N+1 Redundancy is achieved by having one more Uninterruptible Power Supply than is needed to do the job. Therefore, it is possible to use, for example 3×40KVA UPS Systems, or 4×30KVA UPS Systems, that too, can grow with demand. If we take the latter, we would need initially 2x30KVA UPS Systems at a £6,000 outlay. You can add another for another £3,000, and then finally have the last in, at a total cost of £12,000. Of course, this price excludes batteries and installation. However, in this instance you need to have room for 4 UPS Systems!

I have also not included the additional costs of switch gear needed for the standard Uninterruptible Power Supply Solution. So, taking this into account, along with the additional floor space needed, you would have to argue that the Modular UPS would be a good solution.

There is another factor that gives the Modular UPS a wholesale advantage over other methods and that is efficiency. Let us assume for a moment, that the Modular UPS and the Standard Uninterruptible Power Supply, all share the same efficiency at full load. It is clear that UPS systems operating at half load or less will be less efficient. With 2×80KVA UPS Systems on a 75KVA load, each UPS will be operating at 47% load, whereas the Modular UPS with 90KVA of power available, will be operating at 83% load. So there is probably some running cost calculation that you could also take into account.

Money makes the world go round as they say, so if I were looking for simple UPS support, I’d opt for the standard Uninterrupibtle Power Supply, however if I was needing to include some redundancy in there, the Modular UPS is starting to look like a great contender.

Tags: , , ,
Posted in Help Guides, Technology | No Comments »

Overvoltage Protection

Monday, July 13th, 2009

Here’s a neat article written by our friends at APC: Powercuts during summer months may damage data, albeit a confusing title for what the article is about.

They bring up a valid point about overvoltage leading to damage of equipment. Your normal mains supply is designed to operate at 230V±10%, which means a maximum voltage entering your building of 253V. However, the substation providing this voltage has to be able to do so during full power loading. Let’s say you’re on an industrial park and the substation is providing power to all the buildings – the IT infrastructure, the telecom systems, the lighting, the air conditioning, elevators, escalators etc. The load can be quite substantial, but let us take a figure of say, 1000Amps (equivalent to 10 houses). If the impedance on that line was half of one tenth of an Ohm – 0.05Ω the voltage drop across the cable using good old Ohm’s law would be 50V. This means that the substation needs to set its output voltage to around 280V so that when the power reaches your building it is 230V and within limits. However, if the load is suddenly removed – all the air conditioners are switched off, the buildings are empty and nobody is at home, all of a sudden you are hit with 280V, as the lower current causes less voltage to be dropped across the supply cables.

Some people call this a surge and think that surge suppression devices will protect them against it. In fact, this is not a surge but rather a voltage swell or overvoltage condition. (A surge is an overvoltage condition too, but of short duration -usually µseconds), and in order to safeguard your equipment you need to have some form of overvoltage protection. The only way to achieve this is by the use of either voltage regulators or by the Uninterruptible Power Supply (UPS).

A voltage regulator is a (usually mechanical) device that incorporates a tap changing, or continually variable transformer to keep the output voltage to a tight tolerance.

The Uninterruptible Power Supply, however will also provide overvoltage protection by keeping the voltage within limits. How well it does this depends upon the type of technology used:

  • The Offline Uninterruptible Power Supply will provide overvoltage protection by dropping to battery as soon as the mains voltage is out of limits. This will protect your equipment but if this happens regularly or for prolonged periods, the UPS battery will drain and you will lose power.
  • The Line Interactive Uninterruptible Power Supply will provide overvoltage protection by incorporating some voltage regulation. When the mains goes to high, the UPS System will “buck” the voltage downward by changing taps on a transformer. This has the benefit over the Offline UPS System in that there is no dropping to battery for marginal overvoltage conditions.
  • The Online Uninterruptible Power Supply, (aka Online Double Conversion Uninterruptible Power Supply) provides the best possible overvoltage protection. It has a very wide input voltage window, which means it can take very high voltages (as well as very low voltages) without reverting to battery. What’s more the voltage supplied to your system is constant and unchanging regardless of what is happening to the input voltage.

It’s another string to the Uninterruptible Power Supply bow, as not all power problems are as obvious as the power cut. Give your equipment overvoltage protection with a Uninterruptible Power Supply from UPSMart.

Tags: , ,
Posted in Technology, Uncategorized | No Comments »

Remote Access of UPS

Tuesday, May 19th, 2009

I’ve been reading a lot about remote access and monitoring of UPS systems recently. It’s quite easy to do now. In the good old days you had dial up modems that you could call and interrogate the UPS, or the box of tricks could be configured to dial you when it received a problem signal from the UPS.

Now SNMP adapters have enabled the UPS to be connected to your network and can be monitored and even controlled by anybody who has access to that network. Here’s where a lot of the text has been about. Not about the ability to be able to monitor the UPS, but rather the security issue it poses.

A network managers worst nightmare

A network managers worst nightmare

Many systems come with default security settings which apparently many people overlook when setting up the system. What happens is that any hacker with a bit of UPS knowledge gets into your network and not only does he try to plant a few viruses hear and there or extract some data, decides to switch everything off just for the hell of it. A nightmare scenario for any network manager.

I’ve even heard of some clients who are so scared of the possibility that they refuse any communications with the UPS at all, instead relying on the automatic starting of the gen sets in case of mains failure and visible and audible UPS alarms. This seems a bit unnecessary as monitoring the UPS by a web browser is a useful tool and can be secure provided the correct protocols are followed. Just remember that once SNMP is enabled the UPS is part of the network and your network security protocols should cover for this.

Tags: ,
Posted in Applications, Technology | No Comments »

Decentralised UPS Systems

Wednesday, April 15th, 2009

Following on from my previous entry I’ve recently read that Google have lifted the veil of secrecy on their data centres and are now showing the world how they make their DC’s highly efficient. They too, have opted for the decentralised Uninterruptible Power Supply. Well, actually it’s more than that. They’ve actually incorporated the UPS into the server itself. (See this article)

What they do is basically refit the power supply with a battery, intelligent charger and some DC/DC conversion. This makes perfect sense, as you gain efficiency by removing the DC-AC inverter stage as required by all other UPS systems, raising the efficiency from low 90’s to over 99%.

The trouble with this however, is that you can’t actually post fit. You need to have your server built with this technology incorporated at the beginning, and Google custom build their own servers in any case.

There is one big drawback however, and that is they’ve completely ignored power quality. It’s all very well making systems more efficient, but to do so at the expense of power quality seems false economy to me.

An Uninterruptible Power Supply does more than provide battery backup, it should condition the utility power so that any transients, surges, harmonics and all power quality problems are eradicated before they hit your server. Google’s approach seems to ignore this and they may be leaving themselves open to power problems as a result.

I agree with the decentralised approach however, you put the UPS in, as and when needed, saving the upfront costs. Ensuring the UPS are at capacity also has the efficiency benefit. Any problems with the UPS can be easily rectified (and will only effect the server it’s attached too) and probably more importantly, the UPS makes sure that only clean power enters the server.

Tags: , , , ,
Posted in Grapevine, Technology | No Comments »

Poor Power Quality in Europe Costs €150bn

Monday, April 6th, 2009

I’ve recently read a quite comprehensive report by the Leonardo Energy Power Quality Initiative Team, that put the costs of poor power quality in Europe at €150bn, a quite staggering figure.

Not all the costs are due to obvious power quality issues, such as blackouts, in fact, short term interruptions were the main culprit, followed by transients and surges – then blackouts. A rising phenomenon is flicker, and the costs borne here aren’t equipment damage, but rather the effects on individuals working in an environment that is prone to flicker. It’s interesting I find, as mostly power quality cost are put in the “data lost cost” and “hardware costs”, but seldom are people included in the equations.

Flicker is caused by changes in the supply waveform amplitude and is noticeable particularly with CRT’s and lighting. Workers subject to environments where flicker is a problem, complain of headaches, eyestrain and fatigue. What’s more, a lot of people are completely unaware that they have a problem.

Flicker frequencies are relatively low, but fast enough to mean that a line interactive UPS can do nothing about them. In fact, the transformer in a line interactive system may make flicker even worse. The only way to cure flicker, is either to remove equipment that is causing the flicker, remove the equipment that is showing the flicker, or fit an online double conversion UPS system. This will provide smooth power to the systems, eliminating flicker, and of course, protecting against all other power anomalies along the way.

Tags: ,
Posted in Technology | No Comments »

Surge Energy Rating – What’s it all about?

Monday, March 23rd, 2009

I recently had a question regarding the surge energy rating of our MiniGuard UPS systems. The answer is 320J, but got me thinking why anybody would want to know that. So I’ve had a brief scour of the surge suppression devices, with many boldly advertising their huge surge energy capabilities into the thousands of Joules. So the theory is, the higher the surge energy rating the better the device is.

How ludicrous this is, and I’m here to put the record straight. The energy rating of your surge suppression has zero effect on how good this will protect your equipment when a surge hits it. A small surge suppression device will be just as good as a large device for 99.99%* of the transients it’s ever likely to experience, and for the 0.001% of the time you are hit with a large surge as the result of a localised lightning strike, then both surge suppressors will probably fry anyway.

Another thing you should be aware of, is that even though surge suppressor may divert hundreds if not thousands of Amperes of surge current away from your equipment (reading one spec states 40,000Amps, but doesn’t tell you it is only for micro Seconds – that is a few millionths of a second), they will still let hundreds of volts through to your equipment. And guess what? Most of the time it is this kind of let through that will damage your equipment, and it will leave no trace. You will just put it down to random equipment failure. If you’ve fallen for one of these “if your equipment is damaged when using one of our products we will pay for it” guarantees, you will see in the small print that the equipment must have physical signs of damage, or if it doesn’t say this directly, the manufacturer will want to inspect the equipment for such signs before they pay out anything.

So, are surge suppressors a waste of time? Not at all, they provide a useful function in removing the peaks and a lot of energy from large impulses, but these alone will not stop damage to your connected equipment. You need secondary lines of defence, such as filters and/or isolation transformers to really protect your equipment.

*Well Vic Reeves said 88.2% of statistics were made up on the spot, and he’s probably right. This data doesn’t really exist but obtained by careful conservative consideration.

Tags:
Posted in Technology | No Comments »

When is the UPS System actually ON?

Tuesday, December 2nd, 2008

One of the features of double conversion Uninterruptible Power Supplies is that they contain a “Bypass” circuit. This means that in the event of a UPS failure they will circumvent power around the UPS and continue to provide power to the load. One point to note here is that the UPS will continue to provide power even when it may be switched off!

This is different to the operation of line interactive and standby types. You press the OFF button, and then the load is switched off. Press it on again, and there it is, back again.

Now with online systems, generally they will start up in bypass. So you connect power and lo and behold, you have power to the load. Don’t leave it at that!! You must remember to switch on the inverter or you will not be protected.

The UPS will indicate Bypass operation, and will indicate inverter operation when switched on. It’s a feature of online systems that will extend reliability of power to the load, but only if the correct operation of the unit is observed. After all, what’s the point of protecting your system only to leave it switched off?

Tags:
Posted in Help Guides, Technology | No Comments »

Prevention better than cure in protecting healthcare power supplies

Wednesday, November 26th, 2008

By Rob Morris, UK Country Manager, POWERVAR Ltd

Electrical power is an enormous power for good in today’s health environment. It is behind so many of the wonders of modern medical technology, in fact life without it is unimaginable.
While it is true that almost every piece of clinical equipment depends on electricity for life, there is a hidden and dangerous downside – invisible side effects that can damage costly and life preserving equipment – an effect that is frequently ignored. Power problems often have a cumulative detrimental, sometime disastrous impact, and at the very least can shorten the life of vital systems leading to sudden failure and the high cost of repair or replacement.
Our society is so used to depending on the electrical infrastructure it is hard to believe that it is often not only inadequate to provide the quality and consistency of power that sophisticated and sensitive equipment demands, it can actually be damaging it.
While, on the face it, the conventional power supply is remarkably reliable – often available for 99.99% of the time – when power failure occurs it happens at the worst possible time. Healthcare institutions often take care to provide for those rare but crucial occasions when the mains supply is lost. They are less aware of the fact that even when it is available this power carries a great many impurities that may cause sudden damage to equipment and, even if they do not, are causing hidden wear that will ultimately result in the failure of critical components.
These problems with electricity quality can have many sources. Some may be to do with the utility providing the supply and take the form of brownouts and outages etc; others may be closer to home in the healthcare establishment such as overloaded transformers, circuits, blown circuit breakers, and other electrical loads that create disruptive electrical noise, surges, and voltage spikes.
The effects of these problems may not always be immediately visible. Often a vital piece of equipment fails without any obvious cause.
Healthcare establishments and clinical laboratories must take their own steps to ensure that the quality of power is adequate. As in healthcare itself the key is preventative action rather than costly reactive responses.
There are many pieces of equipment available which profess to remove or mitigate the effect of impurities in the supply, others that provide continuity of power in an emergency and some that claim to do both. However, in reality the choices are not always as clear cut as suppliers would claim and the performance of all products are by no means always equal.
Many people have the mistaken idea that an uninterruptible power supply (UPS) will automatically protect against these problems. As we shall see later in this article, this is a mistake. Most UPS systems are only designed to provide backup power in case of a failure.

Finding the culprit
Poor power quality may be the culprit in many cases and the reason that problems are escalating can be found in the equipment itself. Modern instrumentation, analysers and medical equipment rely extensively on the latest high performance microprocessor and semiconductor technology. These devices are extremely susceptible to damage from electrical power transients, noise, spikes and other problems on the incoming AC power line.
With so much of this microprocessor-based equipment running on non-isolated switch-mode power supplies, voltage fluctuations have little effect. Instead, electrical noise, voltage transients, common-mode voltages, and neutral-to-ground problems have become increasingly troublesome.
Electrical transients on the power line can actually punch a hole through a sensitive semiconductor substrate, leading to the immediate failure of a critical component, or they can erode microscopic bits of silicon.
Lightning causes the largest transients; if a spike caused by a nearby lightning strike reaches an instrument through the power or communication cable, it can literally fry the system.
Large value transients can also be generated by electric utility transformer switching, motors turning on and off in the building or even the photocopier in another room.
One of the biggest producers of noise on the power line is an uninterruptible power supply (UPS), such as might be protecting a nearby computer. Excessive powerline noise, of this sort, will have an erosive effect on semi-conductor components. The effect is similar to rust in a car – it may not be visible, have no effect on performance until the day it rusts right through and causes an immediate failure. Electrical equipment subjected to constant electric noise will be affected in the same way and, one day, just stop working.
This same erosion can cause sporadic data disturbances, such as incorrect or unrecognisable characters. And as the components get smaller their vulnerability increases.
Changes in computing power have made the equipment more susceptible to common mode problems. Even systems that are free of spikes and transients can experience common mode problems that corrupt data or prevent systems from communicating with each other.
The problem for medical and clinical applications is that the more sophisticated the technology, the more susceptible it is to power problems.

The cure for common-mode voltage problems
Most sophisticated electronic instrumentation is designed to run only on clean, transient-free, stable, and isolated voltage. Power plagued by electrical noise or branch electrical circuits with high levels of neutral to ground (common-mode voltage) can cause problems such as system lockups, incorrect or unreliable test results, communications errors and no trouble found service calls.
Of all problems, neutral to ground (common mode) noise is particularly troublesome because it is so easily disruptive to microprocessor function but as easily difficult to identify. Fortunately, it is very easy to mitigate with an isolation transformer, which eliminates all neutral to ground voltages and establishes a true zero volt logic reference on the safety ground. Isolation transformers are used in all power conditioners and some UPS systems.

Total protection – or not
A typical building will experience over 6,000 power disturbances a year but only 3-5% of these will be brownouts or power failures. The rest are sags, surges, transients, spikes, lightning strikes, common and normal mode voltage events, and low voltage impulses. All these can be more damaging than a power loss, often resulting in disruption and long term degradation, especially if equipment is not properly protected.
Paradoxically, laboratories, clinics and other institutions invest large amounts in money to reap the rewards of increasingly sophisticated electronic instruments and analysers but very few take steps to protect valuable equipment from damage, eliminate the source of long term erosion or eradicate common-mode voltages for more accurate data acquisition and communications.

The UPSs and downs of total protection
UPS’s are designed to provide backup power and it is important to be clear about what they can and cannot do to protect sensitive equipment. In an online UPS, AC power is constantly converted from AC to DC and back to AC, which ensures that output voltage is well regulated in terms of both frequency and voltage and that the voltage output is free of normal mode (phase to neutral) transients. However, only an online UPS with an isolation transformer will provide the kind of comprehensive protection that most people believe that all UPS systems provide.
Complete power protection includes the following six elements:

  • Surge diverter

    • . This device protects equipment from spikes or transients in excess of 250V. These surges are caused by lightning, switching of heavy loads on power lines, or the turning on and off loads within a facility, such as air conditioning, lifts, switch mode power supplies and AC motors.

  • Isolation transformer.

    • This protects against common mode noise voltage. It blocks disturbances by establishing the vital connection between neutral and ground, ensuring that there is no voltage disturbance at the output of the isolation transformer.

  • Power line filter.

    • This attenuates low voltage impulses, providing a high degree of normal mode protection. It ensures that no disturbing voltages appear between hot and neutral at the output. A power line filter is the one piece of equipment that is missing from most power protection systems.

  • Voltage regulator.

    • This adjusts voltage to meet the precise needs of electronic equipment. While most computers, with switch-mode power supplies, do not need voltage regulation, some instruments and analysers with linear power may need voltage regulation. If so, a tapswitching rather than ferroresonant regulator should be used to prevent new transients.

  • Battery backup/UPS.

    • This equipment is specifically designed to provide continuing power in case of a power failure or brownout.

  • Frequency regulator.

    • Typically found in an online UPS, this device provides constant power by regenerating the AC waveform from a DC device.

Different devices provide different degrees of protection. A power strip surge suppressor will not provide the common mode protection of an isolation transformer. These devices are designed to provide protection only against catastrophic voltage transients. Most are low cost devices that do not last long and often wear out in a few months. This is because they experience the same wear and tear as other electronic components. More expensive surge devices are not necessarily better performers.
A UPS, even an expensive one, may only provide protection outlined in 1 and 5 above. If it has an isolation transformer it will also cover 2, while a high end online UPS will cover all the bases.
It also needs to be remembered that the batteries in a UPS wear out and can need replacing ever three years and, as mentioned earlier, if protecting a computer can generate a considerable amount of electrical noise which in turn may affect nearby instrumentation.
Power conditioners offer 1,2 and 3 protection and if backup protection is not needed then a power conditioner could be a better, more cost effective solution than a UPS.

Powervar Article

Powervar Article

Not all systems require every base to be covered. The key is to know exactly where the issues are and how to design solutions for each individual installation.
Ultimately the challenge for all is to avoid costly disruptions and failures that can delay or prevent a facility from going about its business. Power protection systems can deliver real return on investment once engineers understand what the issues are and can recognise some of the hidden negative effects that come from not protecting their systems effectively.

Tags: , ,
Posted in Applications, Technology | 1 Comment »

« Older Entries