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RedR TSS

The size you quoted appears a little small at first sight. As a rule of thumb, (not strictly technically correct, but good enough for this scenario) you can think of KW (kilo watts) as equal to KVA (Kilo Volt Amps). Adding up all the ratings of the different equipment (The ratings are usually marked on a plate or printed directly on each piece of equipment), this will give a slightly high value. You can then size the generator. KVA= 1xstatic load +2xdynamic load. The largest site load gives the minimum size to be bought.

  • Dynamic loads are: fridge, fan, air conditioner (air cooler?)
  • Static loads are: computer, lights, radio, and charger

I would buy generators of the same size and make for both sites plus a spares pack. A third generator, of the same size would be ideal for complete spare coverage. The vendor/manufacturer should be able to advise you of the contents of the spares pack. This has the advantage that you only need one spares pack and staff training in operation and maintenance can be minimized.

You can choose from manual start, or automatic start-up. They both have advantages and disadvantages.

Manual Start.

Advantages: Cheaper to buy, often simpler, therefore easy to maintain. You can control the load. I.e. chose which devices will be turned off during the generator run times

Disadvantages: Delay in start up can sometimes be catastrophic to some medical processes. Will the same person have to do both sites? There will be periods of no electricity (Power Cut) and there may be periods when the generator is running despite the main electricity supply having been restored. Choose a generator that at least will cut out on the restoration of power.

Automatic start

Advantages: Can provide continuous electrical supply without manual intervention. Minimizes power cuts to users. May be more controllable, useful in the scenario that the clinic does not need 24 hour supply for all devices

Disadvantages: Need to track the fuel usage as it is easy to loose track of the number of hours/fuel used by generator. Control system may be more complex and less reliable. Always buy a generator that can be run in manual mode and choose the model with the simplest control system, preferably, electro-mechanical rather than computer based.

Automatic start generators need to be sized to take full load immediately. Seek advice on the start-up load/current with the manufacturer.

It is possible to reduce the start up load on manual type generators, by going around and turning equipment off until the generator is up to speed, but this is time consuming and prone to errors.

Some general points.

The electricity supply from generators is more stable when the generator is working well within capacity. Larger capacity generators do not use significantly more fuel than a smaller counterpart, so do not be tempted to buy the smallest generator. It is more economical to run say a 25KVA generator at 50% capacity than a 9KVA generator at 110% capacity. The cost of replacing the equipment that would be damaged as a result of the poor electrical supply far outweighs the initial difference in cost of purchase of the larger generator.

The equipment will only take the correct amount of electricity. You cannot put too much electricity into the equipment as long as the voltage rating is correct (i.e.240V or 110v for single phase) so check the voltage rating of the generator. It should match the normal mains supply. And go for the next bigger size of generator that you need as a minimum. You cannot go too big. Larger generators tend to be more reliable. Fuses on the generator should match both the normal supply load and the start-up current- Check with manufacturer.

The generator should be chosen for continuous running rather than short term or ‘Fill –In’/standby. Again check with manufacturer, re longest continuous running time. Can you assume there will be electricity every day? Rule of thumb, take your longest known loss of supply interval and double it (within reason).

For continuous running generators, one of the biggest issues is the fuel tank capacity. You need a large enough fuel tank to run at least for 3 or 4 days without refuelling. The fuel tank should not be in the same building as the generator and the starter batteries. In such extreme temperatures you should also take account of expansion and fuel vapour dispersion. Also safety considerations about whether the generator can be refuelled whilst running or not should be taken into account.

It goes without saying that the security of the entire installation should be taken into account. Generators and fuel are expensive and dangerous commodities.

The supply changeover and generators themselves do not give a smooth supply, so AGC, surge protection and voltage conditioners should be employed over the whole supply as well as protecting particularly sensitive devices such as computers and medical devices.

The operating ambient temperature should be checked with the manufacturer. Also note the minimum temperatures (winter/night time temperatures) as sometimes it is the temperature variations that matter.

Carolyn Crook