help needed filling a domestic installation certificate


Postby davino » Mon Oct 13, 2008 6:47 pm

Hello all..

I filled in a complete installation test certificate today, however there are a few points I am having trouble with, hope somone can help asap here as av got to hand it in to my boss tomorrow..right here goes..

1. under natures of supply parameters: i have all the info for
nominal voltage u/uo
nominal frequency
external ze
number of supplies
but i dont know how to get "prospective fault current"

2.overcurrent protective device: i have all the info for
type bs/en
nominal current rating
but i dont know how to get "short circuit capacity"

3. main switch or circuit breaker: i have all the info for
type bsen
no of poles
voltage rating
current rating
rcd I n mA
but i dont know how to get "rcd I n mS"

4. distrubution board details, i dont know what to put in "Ipf at this board" and "short circuit capacity"

i know this a bit long winded but i really need some help and would really appriciate it if someone can answer this before the evening is over thanks very much guys....
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Postby ericmark » Tue Oct 14, 2008 9:32 am

On a PME supply PFC and Ze are tied together so Ze of 0.35 ohms = PFC of 657 amps.
Short circuit capacity normally number in a little box around 6000
RCD ms is read direct from the test meter you will have tested the RCD 6 times twice at half ma rating one pos on neg then twice at rating at which it must trip I seem to remember at 5 times ms rating or less and then at twice the rating and this is the one that maters and you enter the highest of the two reading. Normally all these instructions are written on the meter.
Ipf = Prospective fault current you have already asked that.
"short circuit capacity" I would use rating of main switch/RCD again normally in box.
If it asks for main fuse details this is entered as code 3 as we are not allowed to break seals and supply authority never tell us what fuse is fitted.
I do wonder what test instruments you have been given you would normally measure.
R1 + R2 and R1 + Rn but latter only used to check volt drop not normally entered on sheet.
The installation resistance of all circuits both the above tests normally with same meter which we normally call by trade name mega.
Then the RCD meter already mentioned.
And then an earth loop impedance meter which will normally also measure PSC often we use an adaptor with these to measure both earth line loop and neutral line loop.
Sometimes all three combined in one meter.
Have you been given test set?
Eric
ericmark

Postby davino » Tue Oct 14, 2008 7:16 pm

hi, thanks for the reply...sorry but bare with me...

if my ze is 0.10 ohms how do i work out pfc?

when you say "short circiut capacity normal number in a little box around 6000" sorry i dont understand what this means...then again you mention short circuit capacity by saying "short circuit capacity I would use rating of main switch/RCD again normally in box." so do you mean enter the rating of the main switch say 100A in the box?

iam using a metrel 2100 easytest, it,
sorry to sound like a simpleton :roll:
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Postby ericmark » Tue Oct 14, 2008 8:35 pm

I see there is another answer but at 21:30 still not got past the Sysop. There are three common short circuit capacities 4700 amp, 6000 amp and 10000 amp.
Even when PSC is measured there is another parameter which is the let through value I have not found any one size fits all on this but in basic terms it means even if you measure the PSC as 10000 amps you may still be able to use a 4700 amp MCB.
As to delay on RCD some (S Type) have a designed delay to give discrimination and you have to read the big red book as to when this is allowed.
With 57 turns to the coil and a degree to my name I still do not understand everything in the 17th Edition. Only a few days ago I found I was miscalculating the maximum amount of cable allowed in a ring main.
If you need fast answers there are enough links to my web sites I am sure you can find my email address.
Yours Eric
ericmark

Postby ericmark » Wed Oct 15, 2008 4:43 am

If ze is 0.10 then 230/0.10 = 2300 amps or more. I say or more because PSC is measured twice once to earth and once to neutral and highest figure used with PME these will be the same.
On screwfix web site http://www.screwfix.com/prods/57795/Ele ... B-MCB-6kA#
Looking at picture of crabtree startbreaker 6A SP Type B MCB 6kA if you click on enlarge picture you can see on breaker side some writing.
Says CRABTREE ENGLAND
BS EN 6******
Then a box says BREAKING CAPACIT ACC*******
Then three boxes first with BS number
Second with voltage range
And third has second box within the box with 6000 in it this is the short circuit capacity which shows these are 6KA which we already know from advert.
I looked at details for your tester and if I have right one it auto calculates the ISC or PSC on the meter.
With the meter I had the PSC was measured Line Neutral and the ELI of course Line Earth but my sons meter we had to make an adapter to measure Line Neutral.
The instructions I down loaded for your meter shows using wandering leads for Line/Loop test seems reasonable straight forward.
Eric
ericmark

Postby davino » Thu Oct 16, 2008 6:53 pm

Thanks for your time its very much appriciated mate...right i have another one and its quite a serious matter.....

tested a tnce system today and the ze was 3.49ohms!!! it should be within 0.35ohms right?? then i tested r1r2 which was within the regs... then i tested zs and it was 3.68 ...
the rcd ramptest wasnt tripping within 30ma and when i tested the rcd it wasnt tripping at all except on the cooker... iam a apprentice but i know this is serious, the installation is a new one with a twin rcd board 25mm tails 16mm earth 10mm gas and water.. its up to the current regs install wise but these tests are way off..
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Postby davino » Thu Oct 16, 2008 7:13 pm

Hi, so whats the setting that calculates the pfc?
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Postby ericmark » Fri Oct 17, 2008 4:50 am

Remember twin and earth uses smaller earth cable so zs – ze = 3.33 ohms = max amps of 69 so max size MCB would be a B10 for this circuit. But volt drop uses R1 + Rn which will be 2.55 ohms don’t know if lighting or power or what it is feeding but seems a bit on high side.
It is common to find faulty RCD’s but I wonder why it should work on cooker these faults should be referred to your Master who ever he is to check.
Eric
ericmark

Postby davino » Fri Oct 17, 2008 6:11 am

The ze i was tested at the incoming supply, the closest point to the manweb side i could get to, if the ze is 3.49 is this to be refered to manweb cos its ment to be within 0.35 isnt it ?
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Postby ericmark » Fri Oct 17, 2008 9:51 am

For TN-S system we expect the Earth loop Impedance to be 0.8Ω or better outside the consumers installation.
For TN-C-S system we expect the Earth loop Impedance to be 0.35Ω or better outside the consumers installation.
On a TT system 21Ω outside the premises and of course you will use an earth rod.
I have come across many premises where the gas and water pipes are the only earth and at first glance one thinks they are TN-C-S but closer investigation shows they are TT also found that on inquiry with MANWEB who are the ones who will tell you what supply you have they have visited site and installed an earth and made it into a TN-C-S supply.
The only way to work out is TN-C-S or TN-S is often to ask MANWEB as we as the user can not always see how they have connected the earth as all sealed.
Eric
ericmark

Postby davino » Fri Oct 17, 2008 9:40 pm

i have contacted manweb regarding the 3.49ohms and there gunna get back to me monday, really appriciate your help,
did you say the metrel 2100 easttest auto calculates the PFC ? if so can you tell me how to do it please .. sorry to be a pest...
thanks
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Postby davino » Sat Oct 18, 2008 6:54 am

i have just been on the mtrel website cant find a download for the manual for my tester , can you tell me where to find it? also i need a simpletons verion of how to test PFC , sorry i know youve gone over this before but i can do it with this tester. didnt you say it has auto test for PFC?
thanks
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Postby ericmark » Sat Oct 18, 2008 3:33 pm

Try http://www.alphatekinstruments.co.uk/se ... entre.html I just picked one at random and from what I could see when you do the ELI test it also displays the PSC current at same time.
Some times different names prospective short circuit or fault or Isc but it is really up to you to find out how your meter works.
Easy to calculate anyway one to other.
Eric
ericmark

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