I have found a serious safety problem with an encased BMU. DO NOT OVERLOAD YOUR BMU, this applies equally to any transformer.
The unit in question is a perspex cased BMU, I stress that this transformer has been subject to OVERLOAD.
The enclosed 1kVA transformer was held in place with glue and expanded foam packing, with no ventilation or method of heat transfer whatsoever.
Every power transformer has a specified temperature rise (above ambient) at full rating, usually 55 degrees Centigrade.
Think about that.. 55 degrees above ambient. Usually a transformer has free air around it that conducts heat away from it.
This is what I found:
I don't know what loading had been used on this BMU but the transformer became so hot that (1) the foam packing has melted, plus most worryingly (2) the transformer insulation and the input wiring PVC insulation has melted, causing a short circuit. The input wiring was held against the top of the transformer by the frankly shocking construction employed.
The unit was causing a trip to operate in the consumer unit (mains supply).
The melting point of PVC insulation is >140 degrees C, so not a great situation.
Unfortunately due to the construction method a visual inspection of similar units is nearly impossible.
The minimum advice for anyone using one of these would be to operate it at nowhere near maximum rating.