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Specifying a new LV main board

Dim-spark

Hi All, 

Context

I'm mechanical by discipline and know enough to be dangerous about "the other side" but I need to put together a brief to get a main LV board replaced, never had to specify one before so feeling a bit out of my depth.

Main breaker is 1600A and is fed from a private sub on private wire HV network. The original Breaker is a Dorman Smith unit (area code doesn't have a 1 in it telling something about age)

The building is office however has some "technical" rooms with more complex requirements. (Parking the clean earth part for a moment).

There is also a "clean supply" which I assume is direct from the sub lv board (which has been contaminated with other supplies now) so we have two incomers.

Having read through the board types, a Form 4 would seem the appropriate category for a board such as this, beyond that I'm not sure what I should be asking for.

In the existing board there are about 10x100A isolators that are spare. 

The building is subject to space being chopped and changed from office to technical spaces so, Ideally I'd want to be able to swap out isolators/circuit breakers with a correct size unit to match the load requirements as and when they present in the future.

Questions:

1). With having two supplies from a private network into the building this seems like bad practice to me due to the emergency isolation risk but I'm correct in thinking there is nothing in the current regs to prohibit this.

2). Having settled on a Form 4, what type should I got for given the need for future flexiblity, given I want to avoid having to take the LV board down to change/add supplies.

3). When would you typically use a Form 4 vs Form 3.

Thanks in advance for the help.

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  • 0

    well the '1 ' got added to the area codes in 1992-1993 ish. So its done at least the first  30 years of its  service ;-)

    The choice of form thing is all about how easily sections can be worked on, while the rest of it is kept energised - the higher numbers having more isolatable compartments, taking up more space and generally being more bosky..

    https://www.sandfordelectrical.co.uk/guide-to-form-type

    clarifies this quite well.
    It  is not the only consideration though - to work on something also needs enough room to get tools and personnel into position safely.
    Sometimes a small number of fused submains each to another panel elsewhere is easier.
    How much of the circuitry beyond the main switchgear is being kept and is there a requirement for earth fault detection ? - As you mention clean earths it may be a funny site with lots of filters, and therefore current wiring in the earth even when operating normally. This will determine the no. and complexity of the outgoing ways.

    Dual supplies is not that uncommon, but needs more thought, and again earthing, or at least the path for fault currents to flow, can get messy.
    Beyond all but assuming the simplest like  for like swap, you will need to involve someone who can answer these questions. it is beyond a simple response.

    Mike.

  • 0 in reply to mapj1

    We have a 3rd party consultant to act as our AP but when I asked him a few questions his first response was sounds like you need a performance spec..... I'd rather be more informed about what I'm asking for hence the questions. Its primarily a board swap with all circuits retained. I'm looking to use MCCB's rather than fuses.

  • 0 in reply to Dim-spark

    Hmm. Perhaps you have the wrong sort of consultant, or the wrong contract, if they are  not involved in requirement capture and you are not that happy doing it.

    Fuses. You may not be aware, but in high PSSC situations, there  can be a significant advantage to have a "death or glory" fuse, or 3, near the origin of any installation of the sort of current you imply, as fuses operate faster at higher fault currents in a way that beyond certain point, breakers with moving parts don't. Then the breakers do not discriminate well with other breakers downstream as it were. In effect the fuse (that never blows really ) will act as a let-through energy limiter, perhaps allowing smaller, lighter and cheaper switchgear down stream of it.
    (similar arguments allow domestic 6kA breakers behind a 100A intake even if the PSSC before the intake is more than 6kA )
    Moving down the branches of the distribution tree towards the loads, MCCBs and then MCBs are entirely sensible.
    Mike

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  • 0 in reply to Dim-spark

    Hmm. Perhaps you have the wrong sort of consultant, or the wrong contract, if they are  not involved in requirement capture and you are not that happy doing it.

    Fuses. You may not be aware, but in high PSSC situations, there  can be a significant advantage to have a "death or glory" fuse, or 3, near the origin of any installation of the sort of current you imply, as fuses operate faster at higher fault currents in a way that beyond certain point, breakers with moving parts don't. Then the breakers do not discriminate well with other breakers downstream as it were. In effect the fuse (that never blows really ) will act as a let-through energy limiter, perhaps allowing smaller, lighter and cheaper switchgear down stream of it.
    (similar arguments allow domestic 6kA breakers behind a 100A intake even if the PSSC before the intake is more than 6kA )
    Moving down the branches of the distribution tree towards the loads, MCCBs and then MCBs are entirely sensible.
    Mike

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