Ask the Expert: BS 7671 Clause 551.7.2 (Additional requirements for generating sets)

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Question:

Can you provide some interpretation and / or clarification on this clause and specifically 551.7.2  Indent (v) of said clause states that the rating of the assembly (where connected via a low voltage switchgear and control assembly) should be the sum of the rated current setting of the incoming circuit protective device PLUS the rated output of the generating set.

The particular situation I have on a healthcare estate project, is that we have a new site wide low voltage switch panel with an new incoming supply (from an adjacent 2MVA transformer) with a OCD of 3200Amps TP&N (commissioned to 2880A TP&N on the ACB), and a 2MVA generator (again commissioned to 2880A TP&N on the ACB) providing the standby supply as per HTM06-02 requirements to a portion of the larger campus.

The specification received is that the generator is to be synchronous with the mains, such that the generator will be tested on load.  

When the primary (TX) supply was to fail, obviously the generator would start and accept the load in the normal manner; but there would be a period of time (albeit short) once the primary supply has been restored, whilst the generator accepted load is synchronised with the restored main supply and prior to the PLC logic closing the motorised protective devices.  

Hence, and in order to comply with clause 551.7.2 (v), I am of the opinion that the rating of the LV switch panel should be 6000Amps.

Would you concur with this interpretation of the clause in this application or is there some mis-information about what this standard refers?  I expect that the clause applies to any means of generation, be it generator (as in this application) or PV array in connected to an LV switch panel?

Answer:

As the generator set is designed to be an alternative source of supply (generally not to be running in parallel with the incoming supply) I do not believe the requirements of this regulation 551.7.2 (v) need to be applied. This set of regulations have been designed to ensure the assembly is sized correctly for parallel sources of supplies such as PV where both grid connection and the PV generator work together to supply the installation.

cef.co.uk

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