Emergency Power System

I think that you may be looking at the problem back to front. I know that some industries/types of building (good examples being a hospital or a nuclear reactor safety system) will have requirements for the back up/emergency supplies, but this will be set as requirements/guidelines for the power supply to be provided to certain services rather than stating the subdivision of a provided emergency supply. The requirements will talk about the independence of the supplies, the required duration and the services to be fed. There may also be differences in the automatic change-over switches as some services may allow manual change over (e.g. a nuclear reactor won't go from safe to meltdown instantaneously so manual change over may be acceptable but a patient on the operating table in a hospital might die in the same time duration so automatic change-over would be more appropriate).

You will need to look at the industry you are interested in to see what requirements are there.

The wiring 'regs' certainly do not. Words like 'suitable for the application' 'good practice' and similar general thoughts abound. However there are often other standards that apply, building control regulations, fire regulations health authority regulations, fuel handling regulations, you name it there may well be a document for it.

But these are rarely specific as to the deep detail of how best to comply - there is no assumption that cable of a certain size or generators of a certain type will be appropriate,  rather the standards tend to indicate what the system must do in terms of  the performance of whatever fire suppression systems, emergency lights etc. that must operate during a power cut - in the sense that they must must allow time for folk to leave the building, or the ventilation must prevent the build up of dangerous fumes until everyone has left; or whatever the desired outcome is.

Much of the inner working of whatever system you care to assemble to achieve those goals is then left as a creative exercise for the design authority.

This is both a great freedom, and something of a curse, as it makes for some very nervous design engineers (I have heard the job described as 'controlled worrying' ), some really groundbreaking new solutions, and on the other hand, quite a few unmitigated cock-ups (plastic cladding on high rise accommodation anyone ?)  There are plenty of guidance documents for those who prefer a near "off the shelf" solution, but very few of these have requirements that are statutory.  The wiring regs  BS7671 for example are not a requirement, and not legally enforceable (law does require systems to be safe and fit for purpose however, and meeting a relevant standard is a good way to show you intended to meet the legal requirements if anyone needs to ask) . It is a very brave decision to depart too widely from the generally accepted practice, and your insurance may charge more, but you certainly can if you have good basis to do so.

I understand that much more of US practice is mandated in the sense of you can only follow the official guidance, with legal enforcement to make sure you do ?

I see. How would this be typically designed/wired in the UK?

Pretty much as you would in the US - although we have very few legally mandated loads (usually fire related systems).


What gets connected is a matter of risk assessment - are you seeing the difference here - we decide what needs to be done based on performance required rather than blind adherence to code


It's also likely we would have less ATS - so a typical system might have a transformer incomer to a switchboard, a bus section switch and a generator incomer to the switchboard - we would use the incoming ACB's and bus switch with interlocks as the ATS - ie swap the whole of the load to emergency supply rather than bits of it. If we have a partial standby, again we might open the bus switch to only support half the switchboard.


As I understand NEC, you would tend to do this at the feeder level via dedicated ATS


Of course we also have systems that keep the mains and generator synchronised


Regards


OMS