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Strima

MCB Breaking Capacities

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Heard this back in Jan when our NIC guru turned up.

If you have manufacturers MCB's in the same manufacturers enclosure, for example Hager in Hager, and they have a breaking capacity rating of 10kA then the actual rating can be increased to 16kA.

Before I go running off and quoting someone's word of mouth as gospel has anyone actually seen this in black and white?

I've been told this is now industry standard and would certainly pull us out of the poo on some jobs as the PFC can be a little high at times.

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Haven't heard this, can't see how it changes the durability of the internals of an Mcb dependent of where it's fitted.

Maybe it's due to Warrenty of parts?

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It was in annexe Za, I think it was in the 17th red book, but cannot say I have seen it in the BGB.

It was to do with the type testing of the assembly if I recall, and partly due to reduced PFC by the connections, I may be wrong on these points though.

Edited by Spark 68

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Could be down to the enclosure and correctly fitted MCB's with spare way blanks etc contains more of the 'blast'?

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Have found this on a forum but my google head isn't working tonight:

The UK has an A deviation to BS 60947 which effectively says that all Customer Distribution Boards used as described above have a conditional prospective fault current rating of 16kA - REGARDLESS of the prospective fault current rating of the individual mcbs or fuses.
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Can’t make my mind up on this but I get the feeling they are pushing for the exclusive use of OEM gear in a board. I can’t see how the housing can affect the maximum breaking capacity. Thermal and instantaneous O/L yes……. But?

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There is mention in the regs of using another back-up protective device on the supply side, 536.1. This allows you use use a lower breaking capaity if required, but still doesn't answer my question...

Why are the regs not so clear cut??? :rolleyes:

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BS60439 Annexe Za.

I have a copy on the PC, I'll put it up when I'm on it next.

There are a few conditions that must be met but essentially all domestic type CUs and MCBs are type tested to 16KA.

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Cheers Lenny, I knew someone here would know.

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HERES THE MEAT.........

"These distribution assemblies are for use on AC supplies, with a nominal voltage to earth not exceeding 300V. The outgoing circuits include short circuit protective devices, each having a rated current not exceeding 125A with a total incoming load current not exceeding 250A.

In the UK, such equipment is referred to as a ‘Consumer Unit’ and as such is covered by this Standard. However, additional requirements from annex ZA of the standard call for the assembly to have an additional test; this is known as the ‘conditional withstand test’. The condition is that the consumer unit must withstand a 16kA short circuit fault when protected by a 100A HRC fuse to BS 1361 type II. Further requirements are addressed by this annex:

• Means of isolation to be via a manual double pole switch disconnector

• Rated current of consumer unit is determined by current rating of incoming device.

• Outgoing protective devices can be MCB’s, Fuses and/or RCBO’s.

* No diversity factors are applicable to consumer units, the incoming circuit and the bus-bar system must be able to carry their full rated current without exceeding the temperature rise limits."

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Awesome stuff mate, will help us out on the ground. We mainly deal with Hager and some Crabtree thrown in for good measure on the streetworks Telecoms power cabinets. They all have domestic CU's fitted but often the PFC is over 10kA which means running around to try and find an MCB to suit.

We do pre-installation surveys but these are normally carried out by telecoms engineers with little understanding of the magical art so we're often caught out on installation day and the company won't allow us to carry spares due to cost, even though they'll happily spend £90 on Flexicon glands that never get used per job...

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This is standard on larger installs, and one reason why some larger DBs have MCCB incomers.

Otherwise, in installs near substations, an MCB wouldn't be suitable for final circuit protection at all.

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I've added the full Annexe Za in the download section now.

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Cheers mate.

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This is actually a good reg/annexe to be aware of, I have seen some otherwise top notch sparks fall down on this aspect, especially when carrying out EICR's and giving the wrong coding.

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Very true, I wasn't aware of this until Jan this year and it would have saved us a few problems in the past, not to mention man hours and cost of revisits.

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