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....