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POWER SYSTEMS


becomes the problem of the maintenance team. The standard design life for health care


UPS installations is 10 years at 20-25 degrees (BS6290 part 4). Other systems may have only five years design life – but for medical systems 10 years is now the norm.


Typical UPS installation Batteries are typically 12 volt cells – usually alongside the UPS, and consist of a string of 10 x 12 volt batteries – 120 volts across the single string.


If a cell fails and goes open circuit then it


can render the whole string inoperative. An installation in a medical facility will therefore often incorporate a parallel string for greater resilience of supply. The most common battery rating is the Amp-Hour Rating. This is a unit of measurement for battery capacity, obtained by multiplying a current flow in amperes by the time in hours of discharge. The ampere hour rating determines how


much current is supplied for how long. For example, in a typical installation, to deliver one hour autonomy the consulting engineer may specify two battery strings, each providing 30 minutes of power in the event of a mains failure. If one fails and goes open circuit then the other string can take up the slack and maintain supply. This type of multi-string arrangement means maintenance teams can isolate one string for maintenance purposes and still maintain UPS integrity. Most in-house maintenance at medical


installations is governed by the standard HTM 06-01 Electrical Services Supply and Distribution: Part B – Operational management relating to Uninterruptible Power Supplies, inverters and batteries The HTM recommendation for UPS


autonomy is 30 to 60 minutes, depending on an assessment of business risk and spacial constraints to accommodate the batteries But the latest electrical wiring regulations


state that UPS installations should offer a minimum of three hours life – unless there is a back-up supply available within 15 seconds with sufficient fuel for 24 hours – in other words a generator. If a generator is available – three hours autonomy can be reduced down to one.


Whatever system a medical establishment


may have in place now to protect critical services, future installations will almost certainly require more batteries because the autonomy requirement is increasing – so certainty in the duration of their performance and maintaining that performance is crucial. Many installations also have a wraparound isolation or by-pass switch which allows a by- pass of the whole UPS – putting the load solely onto the mains power supply. Most UPS installations come with an external by-pass switch arrangement. It is prudent for any operations manager to know how it is arranged, and know how to put the load onto the mains, in the event of a UPS problem.


IFHE DIGEST 2015


Rapid battery ageing The most important factor for battery ageing is temperature. Higher temperatures can actually improve immediate performance but cause rapid ageing and early failure. A temperature of 20˚C is the optimum battery temperature although most systems will tolerate between 18˚C and 24˚C. Lower temperatures are not normally an issue but high temperatures certainly are. At 30˚C the battery design life is halved.


Controlling temperature is vital to avoid premature and potentially very costly replacement, which will invariably lead to system downtime if not caught early enough. It could cause a complete system failure which often becomes evident only when the battery set is placed under load during a mains power supply failure. UPS units generate heat loss and this


increases as the UPS is put under load. Batteries also generate heat when charging, and together the UPS and the batteries can generate a significant heat loss which can be a big factor when trying to maintain an


optimum temperature of 18˚C to 24˚C. The location of the UPS relative to the


batteries is therefore an important consideration and if they are in same room then air conditioning may be required to maintain a safe operating temperature. How the batteries are arranged can also


affect how they perform. If they are tightly packed into a barely adequate space there will not be sufficient room for case expansion and heat dissipation – and so failures are likely to occur. Maintenance teams should be aware of these issues and take appropriate action before they become critical. A worst case scenario with battery failure


can cost tens of thousands of pounds to rectify and leads to extensive downtime. In one incident the site operators at a UK hospital failed to act on warnings about temperature issues and the batteries failed. When the cabinet was opened the


maintenance team discovered that batteries had swollen and burst. Battery casings had melted into a mass of lead and distorted plastic and were leaking acid gel. The


‘A worst case scenario with battery failure can cost tens of thousands of pounds to rectify and leads to extensive downtime.’


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