Nowadays its all about customer minutes lost, once the figure exceeds a certain number (dont ask me what that is) then the electricity supply company are charged by the regulator, so for example a single sainsburys store is a single customer and the electricity company would have longer to get them back on supply than the same equivalent load lost on 1000 individual houses (that are racking up the minutes lost by the thousand) this is one reason that my company dont allow the use of larger transformers than 1MVA (1000 KW) as if a transformer faults it keeps the minutes lost numbers down and 1MVA transformers are plentiful, whereas larger ones might have to be built to order. the other reason is that generators are available with sufficient output, so when a transformer fails a generator is put in place to get the customers back on supply, while the emergency team replace the faulty equipment. in congested areas such as london, there is a facility to link areas via the LV network to get customers back on supply, but this relies on spare capacity at the transformer (and is why we would rarely load a transformer much above 90% of its nominal capacity, the transformer can be run at 110% capacity for a short time, thus allowing 20% to be used to feed a neighbouring area)