PEFC - PSCC the same on a TT

[Gravid]

Setting up a temp site board in a little brick bunker, DNO haven't supplied an earth as its an out house.
So I've installed a rod and am getting a Ze of 44.7 ohms.
Now my PEFC and my PSCC are both 5A's.

Why are they the same as its now TT? Should they be the same .
Should'nt the PSCC be the greater of the two on a TT or not ?
Are there problems with having a really low PFC ?

Whats your thoughts on this, and could someone explain it to me.
Thanks.

 

[simonspark]

Are you saying that your phase to earth fault current is the same as your phase to neutral on a TT system with a resistance of 44 ohms or so on the ZE? This is not possible. Check your meter or buy a new one.

 

[Gravid]

Thats what I thought, I'm going to nip back today and retest. Cheers.

 

[Des 56]

Setting up a temp site board in a little brick bunker, DNO haven't supplied an earth as its an out house.
So I've installed a rod and am getting a Ze of 44.7 ohms.
Now my PEFC and my PSCC are both 5A's.

Why are they the same as its now TT?
Something not right

Should they be the same?
No

Should'nt the PSCC be the greater of the two on a TT or not ?
Yes

Are there problems with having a really low PFC ?
Yes,the mcbs wont like going out when there is a short circuit


Whats your thoughts on this, and could someone explain it to me.
You have either measured incorrectly or your meter is up the spout


Thanks.

Added this because it woldn't post otherwise

 

[pevvers]

Are you saying that your phase to earth fault current is the same as your phase to neutral on a TT system with a resistance of 44 ohms or so on the ZE? This is not possible. Check your meter or buy a new one.

I've seen a lot of this "is your meter (instrument) calibrated/your meter (instrument) must be broken, so change it" lately, and I must say that comments like this leave a lot to be desired. Surely logic dictates that if his measurement of the TT is showing 44 ohms, then this is a reasonable reading for a TT, which would give a PEFC of around 5 amps, which what he has stated, so therefore both these measurements on his instrument appear to be correctly correlating between each other. Therefore, I shall put my neck on the line and guess that either there is an unusual fault, or the PSCC measurement has been taken incorrectly. Test instruments nowadays are extremely robust pieces of equipment, and I cross check around 90 sets every 3 months. I have yet to find any varying readings more than 0.06 of an ohm for loop readings, and these are without doubt due to environmental reasons. Unless the strange odd readings are being found on various jobs, then there's nowt wrong with the test instrument. Rant over. Fire away if you wish!

 

[Gravid]

Ok I've retested I'm getting 1.68 KA now for my PSCC, don't what the hell happened yesterday clearly I tested it wrong. Sorry for wasting your time.

Des when you say MCB's won't like it what do you mean, I thought the rated short circuit capacity of a MCB had to be more than the PFC. Thanks.

 

[pevvers]

Ok I've retested I'm getting 1.68 KA now for my PSCC, don't what the hell happened yesterday clearly I tested it wrong. Sorry for wasting your time.

No worries, always glad to help and I doubted very much that the meter was at fault due to the fact that the internal calculations appeared to be working fine, i.e. 230=5*44 approximately.

Des when you say MCB's won't like it what do you mean, I thought the rated short circuit capacity of a MCB had to be more than the PFC. Thanks.

The way that the MCB needs to function in a TN system is that with the event of an earth fault, i.e. line shorting directly to earth, the current needs to be high enough for the MCB to trip in the required time, i.e. 0.4 or 5 seconds depending on the characteristics. For this to happen, the earth fault loop path (Zs) needs to be low enough to produce a large amount of current. It's simple Ohms law, so that if you have a Ze of 44 ohms, in the event of an earth fault, only 5 amps will flow, therefore this MCB will never trip, and someone will get a nice surprise off some metalwork. To get instantaneous tripping from an MCB, the values given as Max Zs' for the different devices need to be adhered to, otherwise this tripping time will not be quick enough if the Zs is too high. Hence why we use RCD's in a TT system, as there is no adequate low resistance earth path, the RCD takes the role to supply ADS in the event of an earth fault, still using Ohms law, the calculation is 50V=I delta n(RCD tripping current)xR(Zs). As long as the RCD will trip before there is a touch voltage of 50V, then ADS is satisfied, but of course, any TT system with over 200 ohms Ze is unstable, but this is why we can use a 100mA or 300mA S type RCD for ADS for earth faults. A short circuit (between live conductors) will still make the MCB trip as you've stated that the PSCC is 1680 amps, then is there's a short circuit fault then a fault current around that value will flow through the device, tripping it instantaneously.

I hope I haven't babbled too much.

 

[m4tty]

Top answer pevvers nice one mate

 

[simonspark]

I've seen a lot of this "is your meter (instrument) calibrated/your meter (instrument) must be broken, so change it" lately, and I must say that comments like this leave a lot to be desired. Surely logic dictates that if his measurement of the TT is showing 44 ohms, then this is a reasonable reading for a TT, which would give a PEFC of around 5 amps, which what he has stated, so therefore both these measurements on his instrument appear to be correctly correlating between each other. Therefore, I shall put my neck on the line and guess that either there is an unusual fault, or the PSCC measurement has been taken incorrectly. Test instruments nowadays are extremely robust pieces of equipment, and I cross check around 90 sets every 3 months. I have yet to find any varying readings more than 0.06 of an ohm for loop readings, and these are without doubt due to environmental reasons. Unless the strange odd readings are being found on various jobs, then there's nowt wrong with the test instrument. Rant over. Fire away if you wish!

Yeah I take your point. But I have had a Robin meter that one day decided to give the same loop readings every time. Different outlet, different site. Same readings.Bin!
That was what I was thinking when I read the prob. And last year my 6 month old Metrel went a bit funny and saw everything as an open circuit. No, it wasnt the leads.
Also user error did spring to mind, but Im too shy to mention such things

 

[pevvers]

Yeah I take your point. But I have had a Robin meter that one day decided to give the same loop readings every time. Different outlet, different site. Same readings.Bin!
That was what I was thinking when I read the prob. And last year my 6 month old Metrel went a bit funny and saw everything as an open circuit. No, it wasnt the leads.
Also user error did spring to mind, but Im too shy to mention such things

Ah mate, it's not just you, I've seen 2 other posts in the last couple of weeks that the first thing that somebody has said was "are your meters calibrated?", and it's quite rare for it to happen and for you not to be able to notice something odd with the readings. If I had a test instrument that I thought appeared a bit iffy, I would make sure I took some readings from various different circuits to see if there was an acceptable differing of values measured.

Or just buy a Megger, Metrels in my opinion are a bit naff... I had 3 the other day that were reading insulation resistance values of around 87MOhms for a 200MOhm resistor, just because they had been left in a cold van overnight.

 

[Gravid]

You might be right Si, I knew the reading was strange at the time hence looking into it, pretty sure i took two of them, Oh well all sorted now.
Thanks everyone, cheers Pevvers great explanation.