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Clarification on section 536.4.202

Gabriela Ileana Balas

Hi All,

Reviewing the latest document, Section 536.4.202 has raised a lot of concern in terms of the practical implications on projects. My understanding is that if diversity is heavily restricted or not allowed, it means switchgear will need to become significantly larger, or multiple panels will be required to achieve compliance.

I am keen to understand the wider IET community's view on this. Does this rigid calculation apply strictly to all downstream distribution boards? Can compliance be achieved if only the main building panel is sized without diversity, while the downstream DBs include diversity factors?

For example: If I have a 100A distribution board, would this requirement mean I can only supply 10No. lighting circuits of 10A each? Or would a standard configuration such as 2No. 32A socket circuits and 1No. 40A cooker/oven circuit fail to meet this requirement because the sum of the breakers (104A) exceeds the panel rating (100A), even though the actual simultaneous demand is far lower?

This approach could result in vastly oversized breakers, cables, and panels, which is causing quite a bit of confusion during design stages.

Any insights or experiences on how you are handling this would be greatly appreciated.

Many thanks,

Gabriela

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  • 0
    Gabriela Ileana Balas said:
    My understanding is that if diversity is heavily restricted or not allowed, it means switchgear will need to become significantly larger, or multiple panels will be required to achieve compliance.

    I'm not 100 % sure I understand the issue here.

    The requirement is about protecting the whole assembly from overcurrent ... not restricting diversity. Only ONE of the options (a), (b) or (c) in 536.4.202 needs to be met.

    So, for example, if there is a 100 A (maximum) service fuse to BS 1361, BS 88-3 or BS EN IEC 60269-3, and the current rating (InA) of the consumer unit is 100 A, the requirements of 536.4.202 is met, and you are free to use diversity for the installation.

    If there is no overcurrent protection, then (b) or (c) could be used. For (b) this requires a load curtailment arrangement ... typically, see G100/2. For (c) and (c) only, no diversity is allowable.

    If other forms of supply (e.g. solar PV) connects through the consumer unit, things become slightly more complicated, see Regulation 551.7.2.2.

    Note that older assemblies to standards before BS EN [IEC] 61439 series don't have an InA rating, and they would need to be assessed for overall thermal load and busbar rating for additions and alternations - see Regulation 132.16

  • 0 in reply to gkenyon

    Condition a. of 536.4.202 requires that In of the upstream device is less than or equal to the rated current of the assembly (InA) and any outgoing unit (Inc). As I read it, this condition will not be satisfied by a consumer unit, with a rated current of 100 A and protected by a 80 A supplier's fuse, if, as is likely, it contains outgoing devices with a rating of less than 100 A; a 6 A RCBO for example.

    Surely this cannot be the intent of the wording; or am I misunderstanding something?

  • 0 in reply to Geoff Lyons

    or, more correctly, ... outgoing devices with a rating of less than 80 A.

  • 0 in reply to Geoff Lyons

    On further reflection I suppose the outgoing circuit may not always contain a further protective device, in which the wording does make sense.

  • 0 in reply to Geoff Lyons

    This applies to more than just a distribution board. So the assembly could be a series of switches, each of whose current rating is less than the assembly as a whole. The incoming protective device would need to have a lower rating than the lowest rated switch, otherwise it would not be protected.

    I cannot think of an example in a consumer unit.

  • 0 in reply to Chris Pearson

    Indeed, I was being a little blinkered in my thinking Chris.

  • 0 in reply to Chris Pearson
    Chris Pearson said:
    I cannot think of an example in a consumer unit.

    80A (or even 63A) MCCBs in a split-load 100A CU?

       - Andy.

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