TT PFC question

[matt456]

just testing an install for my pending inspection and doing the testing as per onsite guide so do it in correct sequence and im doing my pfc checks and have to low readings,
L to N 0.47 kA acceptable
L to E 93 A is this acceptable and normal?
am i being a doyle and forgetting something here?
its a tt system a fair distance from a village so a good long run from substation

no ideas anyone?

 

[wigan]

L-N takes your breaker out and has to be high enough

L-E will operate the RCD 230/93=2.47 ohms max Zs=1667 for 30mA & 500ohms for 100mA

you have 93amp

Kev

 

[zupos40]

just testing an install for my pending inspection and doing the testing as per onsite guide so do it in correct sequence and im doing my pfc checks and have to low readings,
L to N 0.47 kA acceptable
L to E 93 A is this acceptable and normal?
am i being a doyle and forgetting something here?
its a tt system a fair distance from a village so a good long run from substation

no ideas anyone?

Can I ask what was the supply voltage and what rating and type of protective device at the origin, also what is the largest fuse or Circuit breaker in the CCU.

 

[scotsparky]

Hi Matt

With TT systems you will get low PFC so what you do is RCD everything. Basically the MCB,Fuse it OVERLOAD protection only and the RCD is the FAULT protection.

When testing prove tripping times on the RCD you probbably wont get them on the MCB or fuse.

As for your maximum ZSn values remember to think about your touch voltage which has to be less than 50vAC and use the formula I delta n x Zs <50v

I delta n is tripping curret of the RCD so 30ma in this case

 

[electricAl]

If you`re not familiar with TT`s the PEFC`s can be worrying until you do the maths - then you realise why RCD`s are so necessary with TT.

Assuming single phase @230v then Z-LN (impedance of Line to Neutral) would be 230v / 0.47KA = 0.48 ohms, which is relatively high but understandable due to the run from tx.
If you check out your trip curves in BS you`ll see that 470A is enough to pop all your type B`s (assuming it`s a new install) upto 100A.

Then on the EFLI side...

Ra (aka Ze) would equal 230v / 93A = 2.5 ish ohms - so you`ve done good with your rod(s)

Then, remember Ra x I^n < 50v (safe touch voltage)

So your Ra (Ze) of 2.5 x 0.030A = 0.075 (a very lowly 75 milli-volts - a very safe touch voltage )

Even if you`ve kept the 100mA front ender for some ccts that`ll give 250mV (2.5 x 0.100)


Understandable that your anxious for your assessment, but on those figures you can relax a little

 

[scotsparky]

Matt just a quick question when you took your ZE did you have a full shut down and isolate the installation take a true ZE or is it a ZE(DB)?

If so you will need to prove your rod reading as you have got earth paths throught your pipework and metalwork which may give you an falsly low reading.

 

[electricAl]

Ah bagger, just realised 3 of you have already replied...

Sorry for repeating what you already said

i fink that last drink has gone to me burp head

 

[scotsparky]

electric your post got me thinking about why the Ra readings were sow low on a tt

 

[electricAl]

Totally wi` you big Scotsman.

a bit out of balance readings considereing Z LN so high but suspiciously low Ze

Tink your question well worth asking - don`t want the guy to mess up due to what i said, sure hyour the same

 

[matt456]

Matt just a quick question when you took your ZE did you have a full shut down and isolate the installation take a true ZE or is it a ZE(DB)?

If so you will need to prove your rod reading as you have got earth paths throught your pipework and metalwork which may give you an falsly low reading.

hi,
i tested the installation with the earth attached to the cu, then i tested the earth electrod resistance which is 120ohms
(it has been raining quite heavily )
voltage at source is 246V

Can I ask what was the supply voltage and what rating and type of protective device at the origin, also what is the largest fuse or Circuit breaker in the CCU.

voltage is 246
largest mcb in cu is 32A
boards fuse 1361 type 11a 60A
cheers matt

Also which reading do i put for pfc on the test sheet?
thanks everyone for their help!
matt

 

[electricAl]

Matt, are you saying it had been raining heavily when you got 120 ohms ?

& who`s the inspection with ?

Did you follow the explanations of the numbers you`d got?

 

[matt456]

inspection is with niceic!
it had been raining heavily, just had a look where the stake is there is a weed sheet underneath gravel so it is dry under there!
Yeah ran through the calcs did them myself to get up to speed with it again
which value do i record for the pfc the L-n value?

 

[scotsparky]

L-E value

 

[matt456]

cheers matey

 

[electricAl]

Is it not the case that the NICe insist on 100 ohms or less then?

Not with em meself but i `thought` that was the case - is there a bit of leeway shown - i.e. must be below 200 but `like` you to achieve 100 or less


& as Scotsparky said it`ll be your PEFC you`ll record in this case - cos that`s the greater of the 2 PFC `s

 

[pevvers]

L to N 0.47 kA acceptable
L to E 93 A is this acceptable and normal?

Surely it would be PSCC that is the highest, at 470A, not the PEFC at 93A? Or have I missed something silly....

 

[electricAl]

Surely it would be PSCC that is the highest, at 470A, not the PEFC at 93A? Or have I missed something silly....

No, i don`t think you have

You`re quite correct if you`re saying that the PSCC will be filed as the PFC.

The OPs confusion was due (i`m assuming) to the very low PEFC values TT installs throw up. Hence the discussion

or atleast i thought thats what we were discussing

 

[pevvers]

Oh, it was only because he was asking which one to put down as the PFC value a couple of posts back, and you both said the PEFC.... Didn't want him to get a cross against something in his inspection by putting down the 93A value on his paperwork. Although with a Ra of 120 ohms it's gonna be a lot lot smaller than 93A.

I'm think I'm confused by the whole discussion, if I am please just ignore me and I'll go sit in the corner.

 

[electricAl]

Oh, it was only because he was asking which one to put down as the PFC value a couple of posts back, and you both said the PEFC....
I'm think I'm confused by the whole discussion, if I am please just ignore me and I'll go sit in the corner.

Umm... don`t think anyone did actually

Maybe i`m confused -think i`ll join you in the corner

 

[pevvers]

Ah bugger it....

 

[scotsparky]

I said the Phase earth fault which is the PEFC but dont worry about it.

 

[matt456]

i was looking for which value to put in the pfc box on the IET websites EIC certificate
0.47 ka was the reading L-N and 93A was the reading L- E
im a little confused as its been a while since i tested a tt system so hence a little confusion over the readings!!

 

[pevvers]

Then you would put the higher of the two, which would be the 0.47kA.

 

[scotsparky]

on the Zs and PFc boxes i put in the Higher Ze and Lower PFC as that is the "worst case sinario" as such if that complys all others do.

 

[pevvers]

Ah, so that's where the confusion has come into the discussion... On my part anyway.

I was always taught to put the highest of the PFC's into the box!

Would the worst case scenario not be the higher of the two?

 

[scotsparky]

Worse case woulf =ld be High ZE and low PFC remamber you want a high fault current for your disconnection times. so if its lower then slower disconnection times.

 

[pevvers]

I understand what you're saying completely, but I am going by what is written in GN3. I only have the 16th edition copy with me at the mo, but it states that "Prospective fault current (PFC) - The value recorded is the greater of either the short-circuit current or the earth fault current. Preferably determined by enquiry of the distributor"

Damn, I sound really arsey for a new poster on here but I don't mean to be. But like I said, I don't want the fella to get marked down on his NIC assessment.

 

[Lou7]

Record the highest reading, double for 3-phase,or x1.74 as I was told by NICEIC.

 

[electricAl]

Actually its 1.732 - the root of 3

 

[scotsparky]

yes fair play no worries Its just thats what i do i do stant to be corrected though. Mind you i also douple the PFc for 3 phase.

 

[pevvers]

Yeah, I've always been told just to double it for 3 phsase, never heard of anyone wanting it as accurate as taking decimal places into it....

This installation is single phase though.

 

[electricAl]

Doubling it is fine, always was & always will be. It errs on the side of caution.

But as someone was being more precise - i was, well, more precise

Not like me really

 

[matt456]

so the reading i need to put in the box for the pfc is the lower value of 93A?

 

[pevvers]

Well, if you go by GN3, then it will be the higher, i.e. the 470A from the PSCC (0.47kA).

Honest!!

 

[Lou7]

The reason I used the 1.74 was that I had a reading that put me over, and was told that I could use that instead of doubling,giving a lower result. I thought it was 400 divided by 230


Record the highest.


or was 415/240 ?

 

[pevvers]

The reason I used the 1.74 was that I had a reading that put me over, and was told that I could use that instead of doubling,giving a lower result. I thought it was 400 divided by 230

Yup, completely agree with that as the doubling is only a rule of thumb as it's simple to do on site. I must remember that in case I get any borderline results someday...

Why would it be root 3 electricAL? I'm always looking to learn new stuff... Root 2 I've heard of, but never 3.

 

[electricAl]

Yup, completely agree with that as the doubling is only a rule of thumb as it's simple to do on site. I must remember that in case I get any borderline results someday...

Why would it be root 3 electricAL? I'm always looking to learn new stuff... Root 2 I've heard of, but never 3.

The root of 3 is intrinsic with TP systems.

It`s the maths behind how we derive 400v between phases from a 230v to earth supply...

230 x root of 3 (1.732 is near enough but it`s really 1.73205081) = 400

Works in reverse too, of course... 400 / 1.732 = 230

Originally it was 240v x root 3 = 415v

 

[scotsparky]

400 devided by root 3 = 230

 

[matt456]

thanks to everyone who has helped me!
new question coming soon

 

[electricAl]

400 devided by root 3 = 230

Harhar...beat ya too it

Is it just the eld uns that know this kind of thing?

 

[scotsparky]

the young ones are taught it but if anything like me it takes a few years to sink in. The young one are to busy thinking about what they did to the young girls on sat night. I was

See pic on how we get the figures.

 

[electricAl]

the young ones are taught it but if anything like me it takes a few years to sink in. The young one are to busy thinking about what they did to the young girls on sat night. I was

go on, go on...we`re all dying to know!

you was what?



Personally couldn`t wait til Saturday night - Friday came sooner

Arhhh, memories, mamories. Such a long time ago...

 

[scotsparky]

gave more info not bad for a monday night!

 

[pevvers]

I'm pretty sure I must have learnt it, or something along those lines in my HNC, but that was 6 years ago and I haven't used anything in it since.

Sounds about right though, cheers for the info,

 

[scotsparky]

well i remebered it from 18 years ago

 

[pevvers]

Good for you!