Prospective fault current

[Mike Me Mate]

Hi I'm looking for some advice and options. I have installed a new board a British general one the customer has bought. I also got the meter cut out upgraded with a main switch put in.

At the main sw the pscc is 17.8, pefc is 6k
Which seems funny to me cause tncs which I'm sure new cut outs to be should be the same.
It's a 100a 1361 typeII cut out fuse.

Any way at the board the pscc is 8.7 and pefc is 6.

The easy way to deal with it was to put in a10k breaker. However bg or Mk sentry do not make 10k mcbs. Is there any 10k mcb that fitt Mk sentry boards or is there another option?

 

[malcolmsanford]

Your type II BS 1361 will deal with the PFC as it it rated 33Ka, so just keep the 6Ka breakers,

How did you get a PEFC at the isolator do you have you main earth going through that ?

I'm also a little concerned with the 1.7Ka difference you have if it is a TNC-S system, are you sure it is ? Though not a clear indicator, but a pretty good guide what was your Ze?

 

[Mike Me Mate]

I have a mft1502 tester. Manufactures instructions say to do a 2 lead test.

 

[malcolmsanford]

I have a mft1502 tester. Manufactures instructions say to do a 2 lead test.

OK Mike and I assume you did.

 

[Mike Me Mate]

Oh I got a henry block as a main Earthing terminal. I just uses it as a reference.

My ze measures out .06 at the switch
0.06 at the board funny readings eh? Doesn't really work out with calculations.

I tried to open up the new meter but I couldn't find any screws to take it apart on the neutrals side. Do you think his main earth connection is loose? Why would a meter guy do a tns connection on a new meter? Another thing that was quite odd is that he put 16mm tails for a 100a main fuse, from the Cut out to the main switch.
This install is going to be used as a job for my napit scheme and has to be perfect. I'm still a little worried about PFC numbers at the top of the certificate and having a 6k breaker. Just doesn't seem right.

 

[Mike Me Mate]

Sorry I meant opened up the cut out.

 

[malcolmsanford]

Oh I got a henry block as a main Earthing terminal. I just uses it as a reference.

My ze measures out .06 at the switch
0.06 at the board funny readings eh? Doesn't really work out with calculations. Your calculating that the PFC should be 3.83Ka I assume. Remember that our MFT is not designed to work to such precise tolerances and so with a pretty low Ze of 0.06 there will be discrepancies. On a post here our learned friend IQ gives details on how you can carry out a Ze test using a wander lead to bring up the tolerances on your meter.

I tried to open up the new meter but I couldn't find any screws to take it apart on the neutrals side. What ever you do do not open the meter, or do you mean head.

Do you think his main earth connection is loose? No

Why would a meter guy do a tns connection on a new meter? Again I think you mean head. Most new heads are generic TNC-S types, so he would fit it and adapt it to the earthing system available.

Another thing that was quite odd is that he put 16mm tails for a 100a main fuse, Have you physically checked the fuse in the head? is it 100amp? from the Cut out to the main switch.

This install is going to be used as a job for my napit scheme and has to be perfect. I'm still a little worried about PFC numbers at the top of the certificate and having a 6k breaker. Not sure what forms you are using but on the EIC form in the BS 7671-2008 there is a section about supply characteristics and earthing arrangements in the section nature of supply characteristics you would put the 33KA for the BS 1361 Type II. Just doesn't seem right.

Hope this helps

 

[tony mc]

This is the post that Malcolm makes reference too.

Higher-accuracy low Ze test procedure

 

[IQ Electrical]

As Malcolm has already said, the BS1361 II (or BS88-3 as it is now called) will give back-up protection to your BS60439 consumer unit and MCBs/RCBOs.

This back up protection is only to be used on PFCs of up to 16kA despite the device being rated at 33kA.

So your consumer unit is covered as your PFC at that position was only 8.7kA and anything else upstream of that is covered by the 33kA rating of the suppliers fuse.

Point your NAPIT chap to BS60439-3 UK Annex ZA if he queries this scenario although I'm sure he'll be fully coversant with that British Standard

 

[Mike Me Mate]

I seem to come across this problem commonly enough. Really high pscc.
I need a standard rule of thumb.
We have several different cut out protection device.

Bs1361 type I 16.5k Back up protection =

Bs1361 type ii 33k BU pro = 16k

Bs88 part 6 16.5k BU pro = ??k

Would you generally say that the back up protection is half? Is this backed by the scheme partners?

Thanks.

 

[Chr!s]

Well you need to to look at the consumer unit to see if it comply s with uk annex za.

Secondly if your relying on the bs7671 for back up protection then you need to realise that you can not have discrimination between any of the devices.

The bs7671 will limit the fault current, if the CU is of the appropriate type then this will allow the mcbs to be rated at 16ka in effect, there are some limitations, if you don't have uk annex za, GN3 has a little more info. One point being the mcb's are limited to 50 amp.

As for back up protection, its not generally half it depends upon the level of fault current and the device.

Regards Chris

Regards Chris

 

[Chr!s]

Just to add mike, at a high pfc, both fuses limit current to a great degree, around 9 ka for both fuses at 16ka, at lower pfcs there may be very little current limiting.

Regards Chris

 

[IQ Electrical]

Just to add to what Chr!s has posted, the 16kA conditional rating is the value that the consumer unit and components must be able to withstand during the tests described in UK Annex ZA.

I've attached the Annex for any insomniacs.

 

[Mike Me Mate]

Ah yes! The guidance notes are very clear! I would also like to point out that in addition to the 50amp limit the consumer unit has to be supplied through a type 2 to bs 1361:1971. very interesting! Now since it's supplied with a bs88 1967 (only a few around) in which western power can not tell me what the k rating is because it so old. I am on them to update it for free to a type ii. So it could comply.

But I do understand that there will be no discrimination in the case such a large current comes through.

So I guess in everyday language what would happen is the mcb/rcbo would break/fuse but the main cut out cut off the supply before any real damage happens according to the annex notes. Correct?

 

[Mike Me Mate]

Just to add to what Chr!s has posted, the 16kA conditional rating is the value that the consumer unit and components must be able to withstand during the tests described in UK Annex ZA.

I've attached the Annex for any insomniacs.

In the case that any monitors are looking at this post. Using the iPhone application tappa talk does not let me view any attachment and comes up with a restriction page. Just though I let you know!

 

[IQ Electrical]

Ah yes! The guidance notes are very clear! I would also like to point out that in addition to the 50amp limit the consumer unit has to be supplied through a type 2 to bs 1361:1971. very interesting! Now since it's supplied with a bs88 1967 (only a few around) in which western power can not tell me what the k rating is because it so old. I am on them to update it for free to a type ii. So it could comply.

But I do understand that there will be no discrimination in the case such a large current comes through.

So I guess in everyday language what would happen is the mcb/rcbo would break/fuse but the main cut out cut off the supply before any real damage happens according to the annex notes. Correct?

That's the theory!

It'll be interesting to see if the supplier will change the fuse, we find that the mention of an occasional burning smell from the service head area tends to encourage them

 

[Mike Me Mate]

They just got back to me. Its a pretty unclear area for them. They replace the rewirable fuse heads for free. However they seem to want to charge for this one. Technically though they replace the rewirable because I could guaranty a k rating on it cause it didn't say sa1,2,or3. Well in my eyes if they can't give me a rating for this bs88 1967 fuse then? What's the difference?

We then had the discussion of changing the fuse carriages directly over from the bs88 1967 to type 2. He doesn't know but he said if it can then they'll do it for free.

On another note. The rcd. They don't have to have a k rating? Rcd for dual splits don't trip on overload. They work on leakage(earth fault) 30 m/a is 30 m/a. Is there anything to be wary about this?

 

[malcolmsanford]

Yes making sure they trip .....................what do you mean by wary?

 

[Mike Me Mate]

Yeah stupid question sorry. I guess my point is that for a short circuit current the rcd doesnt activate because of there is in theory no imbalance between live and neutral Now if there is an earth fault the PFC will never rise that high because it would cut out at 30ma. So why do they bother putting k ratings on rcd, noticing that they don't put them on mainswitches.

 

[SPARTYKUS]

Yeah stupid question! Sorry!

Hello Mike Me Mate :winkiss:

No such thing in my book (stupid question) although in this industry and with the way it is, you may occasionally get a tongue lashing from someone.... then the thick as a rhino skin needs to play its part.

Even if you are 30 years man and boy there will be something you do not know, or have even, god forbid, perhaps forgotten. It's easy to forget that sometimes though as we are all fickle creatures.

Good luck with the assessment.