Form 4 - Overcurrent Device - What to put down

[chrisgc]

Hi

On the form 4 - in the brb - in the box entitled:

Overcurrent Device 4 Short-Circuit Capacity: ........kA

What is that referring to?

Is it the RCD in the cu?

Just wondering as this box is above the sub headings of type and rating - which I know are for the circuits individual breakers.

Is the Overcurrent short capacity box for the RCD of the CU?

If it is what do I put down if only half the circuits in the cu are protected by a rcd?

 

[telectrix]

overcurrent device is the MCB and the s/c capacity is usually 6kA. found on side of MCB as 6000

 

[SirKit Breaker]

The S/C rating is also on the front of the breaker in an oblong box, it will say 6000 or 10000, this is the short circuit current rating that you require. Just in case you are not aware, this figure must be at or above the PSCC of the installation you are fitting it to.
Most domestic applications will be below 6kA.

 

[chrisgc]

overcurrent device is the MCB and the s/c capacity is usually 6kA. found on side of MCB as 6000

If its for the MCB's on the circuit (obviously more than one in the cu) and Form 4 enables you to have different circuits listed (down the side of the form) on the sheet and the MCB's are different - thus different ratings - in the CU how does the box titled --Overcurrent Device 4 Short Circuit Capacity ........kA (which is above the Type and Rating headers)-- cover all the MCB's on in the CU??

Also dont think I can see the ratings etc as the CU i have in my house has, circuit breakers like this: NB30 30a - type 2 240v on the front of them.

So a little confused and thought that the part of the form I am on about would be different for different cb's.

 

[telectrix]

sounds like the older type plug in MCB's used to replace rewireable fuses e.g.wylex. if i rem. correct the s/c capacity was 3kA, stamped on side of MCB as 3000 in a box

 

[Graeme Harrold]

It can be a pain in the rear with some of the older ones as they were only stamped on the side. Dont think Ive had a domestic with PSCC above about 2.5kA. End of the day mate, its your signature on the sheet, and you are going to have to look. Power off, busbar off and ping them out 1 at a time.

 

[SirKit Breaker]

sounds like the older type plug in MCB's used to replace rewireable fuses e.g.wylex. if i rem. correct the s/c capacity was 3kA, stamped on side of MCB as 3000 in a box

You are right mate, i forgot about them and they are also BS 60898 as well

 

[chrisgc]

sounds like the older type plug in MCB's used to replace rewireable fuses e.g.wylex. if i rem. correct the s/c capacity was 3kA, stamped on side of MCB as 3000 in a box

Just had a search and yes older type.

So the box on Form 4 - I still am unclear what I put as its at the very top of the column and appears as though it would cover all the Cb's in the CU??? Any help please?

 

[telectrix]

whatever the S/C rating is on each MCB, in each box alongside the type, rating, etc. should be same for each one

 

[IQ Electrical]

If it is a type 1,2,3 or 4 then it's a BS3871 and can have a s/c capacity anywhere from 2-10 kA.

Remember, in a domestic situation on a consumer unit to BS60439-3 you can rely on the s/c capacity of the DNO fuse which is taken to be 16kA.

 

[WayneL]

If it is a type 1,2,3 or 4 then it's a BS3871 and can have a s/c capacity anywhere from 2-10 kA.

Remember, in a domestic situation on a consumer unit to BS60439-3 you can rely on the s/c capacity of the DNO fuse which is taken to be 16kA.

This is what I was told by an assessor.

I originally thought it was the Short Circuit Capacity of the MCBs that was recorded in this box - but he said it wasn't, it's the rating of the cutout.....e.g. 16 KA

 

[chrisgc]

This is what I was told by an assessor.

I originally thought it was the Short Circuit Capacity of the MCBs that was recorded in this box - but he said it wasn't, it's the rating of the cutout.....e.g. 16 KA

WayneL

Where on the cutout do you find that info????

 

[WayneL]

WayneL

Where on the cutout do you find that info????

Hi mate,

Get the info from 'Table 7.4' of the On-Site Guide (Pg 57).

 

[chrisgc]

Hi mate,

Get the info from 'Table 7.4' of the On-Site Guide (Pg 57).

WayneL

Sorry I meant where on the actual Cutout can you find the type?

 

[IQ Electrical]

The information is on the fuse itself but of course you can't see it because it's sealed.

Then again, you could just isolate all load, cut the seal, remove the fuse, record the details and put everything back neatly but that would be illegal.

 

[chrisgc]

The information is on the fuse itself but of course you can't see it because it's sealed.

Then again, you could just isolate all load, cut the seal, remove the fuse, record the details and put everything back neatly but that would be illegal.

So that would be the fuse with the already broken tag on then lol.

Come to think of it where can you get the metal tags which have accidently on purpose been cut??

 

[IQ Electrical]

Maybe a well-known auction site?

 

[telectrix]

loads on ebay. but some are flimsy and cheap cack. but you got to put something on when you find a broken one!

 

[chrisgc]

Hi again

I have managed to view the MAIN FUSE and it has the following markings on:

100A
ASTA 20
BS 1361

Would I be right in saying that the info I would put down in the box/section I am on about (Overcurrent Device - Short-circuit capacity.........kA) is in regards to the MAIN FUSE and NOT the individual circuit breakers???

I just want it clarified thats all.

Also how do I go about finding out the kA of this as thought the max for domestic would be 16.5kA but unsure where to found this info in relation to the main fuse I have?

 

[pushrod]

This is what I was told by an assessor.

I originally thought it was the Short Circuit Capacity of the MCBs that was recorded in this box - but he said it wasn't, it's the rating of the cutout.....e.g. 16 KA

Well for my 2pennyworth i think you were right and the assessor was having a bad day or talking out of his a***!

Note 4 on P141 of the OSG tells you that it is the mcbs by referring you to table 7.4!

We were taught that you check all the short circuit capacities on the mcbs and if they are different you record the lowest. If that is not greater than the PFC there is a problem because it means that a fault current could cause an mcb to explode!

You then have to sort it out - either upgrade the offending mcbs (maybe 6kA to 10 kAs), or put them in an enclosure that was rated to withstand that size of pfc or investigate lowering the pfc .

Seemed to make sense to me.