Hi all. I've just completed my first re-wire and everything is fine with the testing with one exception. When I measured Ze I got a value of 0.01 ohms. The distributor changed the supply over to PME and the means of earthing is TN-C-S. The problem is a value of Ze of 0.01 ohms equates to a PSSC of 23KA!! PEFC is 2.7KA. Can anyone help me work out what's going on? All the rest of the testing results where normal but this one is confusing me. I doubt I'm going to find breakers that can take 23KA and still work afterwards..
PFC Question
- Thread starter cr0ft
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Help Support Talk Electrician Forum:
have you actually measured PFC? or calc it?
DNO usually guarantees PFC of <16KA for normal domestic on TNCS
DNO usually guarantees PFC of <16KA for normal domestic on TNCS
No, the supply is TNCS with PME as the system of earthing,
the other point is this,
L-N and L-E should be identical on TNCS, its the same cable!
Maybe I'm measuring it wrong but I don't think so. The instrument is a Dilog 9083P, three probe test meter. for PSSC I clip neutral probe to neutral and earth probes to neutral busbar. Line probe goes on incoming line connection to main switch. Press button and measure PSSC.
Then I repeat but move the earth probe across to the earth bar and measure PEFC.
I got a value of 0.01 ohms impedance for PSSC (23KA) and 0.09 ohms impedance for PEFC (2.7KA).
PS Ze was measured as 0.01 ohms. I disconnected the earthing conductor from the earth bar and clipped the earth probe of my tester to it. Line probe to incoming line on the main switch and neutral to incoming neutral.
---------- Post Auto-Merged at 17:04 ---------- Previous post was made at 16:50 ----------
What would you guys do if you got these results? I'm thinking of following it up with a call to the DNO to ask them to test Ze more accurately. The main fuse is a 1361 which I believe only has a breaking capacity up to 16KA.
Then I repeat but move the earth probe across to the earth bar and measure PEFC.
I got a value of 0.01 ohms impedance for PSSC (23KA) and 0.09 ohms impedance for PEFC (2.7KA).
PS Ze was measured as 0.01 ohms. I disconnected the earthing conductor from the earth bar and clipped the earth probe of my tester to it. Line probe to incoming line on the main switch and neutral to incoming neutral.
---------- Post Auto-Merged at 17:04 ---------- Previous post was made at 16:50 ----------
What would you guys do if you got these results? I'm thinking of following it up with a call to the DNO to ask them to test Ze more accurately. The main fuse is a 1361 which I believe only has a breaking capacity up to 16KA.
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Technician
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Could you get another guy in to do the same read with another
type of meter?
16kA is (usually) the maximum for domestics as one of the
others has said.
You MAY have a meter issue.
I had a di-log and I never had this kind of thing.
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I think it's an issue of meter accuracy as I believe most 17th Edition test meters are only accurate to within 0.05 ohms or so. An increase of just 0.01 ohms for Ze would mean PFC reducing to 11.5KA.I don't know any other electricians locally to be honest so I think I will call the DNO and ask them to check it out. If the PFC is under 16.5KA then the main fuse will break the worst case fault current assuming the MCB doesn't.
---------- Post Auto-Merged at 17:17 ---------- Previous post was made at 17:16 ----------
@ Canoeboy - I have a copy in front of me just now. Specifically?
---------- Post Auto-Merged at 17:17 ---------- Previous post was made at 17:16 ----------
@ Canoeboy - I have a copy in front of me just now. Specifically?
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I think it's down to the probes not getting a good contact.
TBH I'm never happy with 3 wire tests, because I'm not 101% sure always exactly what it's reading.
To be sure, test with E and N probes on N (pssc), then test with E and N probes on E (pefc)
They should be identical readings, as steps has already pointed out, they are the same wire connected to a common terminal
TBH I'm never happy with 3 wire tests, because I'm not 101% sure always exactly what it's reading.
To be sure, test with E and N probes on N (pssc), then test with E and N probes on E (pefc)
They should be identical readings, as steps has already pointed out, they are the same wire connected to a common terminal
Dave, exactly what I was going to suggest,
if I ever get an inconsistent reading then I put both probes on the same conductor,
does your di-log do a 2wire test? [as already suggested]
if I ever get an inconsistent reading then I put both probes on the same conductor,
does your di-log do a 2wire test? [as already suggested]
Hi and thanks. It only does a 3-wire test but the method for PEFC which gave 2.7KA is red to incoming line on main switch, black to neutral busbar, green to earth bar. For PSSC it's red to incoming line again, black to neutral busbar and green to neutral busbar.
I'm using crocodile clips to connect to busbars of course. I took the reading a few times and it kept coming up with the same result. Somewhat strange.
---------- Post Auto-Merged at 18:50 ---------- Previous post was made at 18:47 ----------
Just a thought, won't parallel paths increase the PEFC reading? Both tests are of course done with the earthing conductor connected so I would expect PEFC to be higher than PSCC.
I'm using crocodile clips to connect to busbars of course. I took the reading a few times and it kept coming up with the same result. Somewhat strange.
---------- Post Auto-Merged at 18:50 ---------- Previous post was made at 18:47 ----------
Just a thought, won't parallel paths increase the PEFC reading? Both tests are of course done with the earthing conductor connected so I would expect PEFC to be higher than PSCC.
OK, the way I see it is this,
PSCC is the largest current that can flow, regardless of the conductors involved,
do a PFC with both your probes on N, then repeat with both the probes on E,
a short can occur just as easily to E as it can to N , I dont see the difference.
---------- Post Auto-Merged at 18:53 ---------- Previous post was made at 18:51 ----------
you have already stated it is PME,
even at simply TNCS the N and E conductors on the DNO side are the same conductor.
PSCC is the largest current that can flow, regardless of the conductors involved,
do a PFC with both your probes on N, then repeat with both the probes on E,
a short can occur just as easily to E as it can to N , I dont see the difference.
---------- Post Auto-Merged at 18:53 ---------- Previous post was made at 18:51 ----------
why?
you have already stated it is PME,
even at simply TNCS the N and E conductors on the DNO side are the same conductor.
How would you deal with it? Would you just note the high PFC and state no action taken due to meter tolerance levels on the certificate?
---------- Post Auto-Merged at 22:11 ---------- Previous post was made at 22:08 ----------
If I'm reading GN3 rightly (pg 54), where the service cut-out contains a cartridge fuse to BS1361 Type 2 .... the short circuit capacity of the overcurrent protective devices within consumer units may be taken to be 16KA. As you've rightly said, the Type 2 fuse is good for 33KA but it doesn't seem that GN3 allows the consumer unit MCB breaking capacity to be uprated?
sparkytim
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It is unlikely that the PFC is really that high
Most DNOs state that the max PFC WILL be 16Ka due to internal factors on their supply transformers
It has been suggested that taking an end of line Zs and deducting the R1 + R2 will give a better indication of the true Ze and then calculate the PFC based on that.
Also remember that the certs. state that Ze and PFC can be obtained by enquiry
If you enquire then you will be given Max 16 KA and 0.35 ze for TNCS
Most DNOs state that the max PFC WILL be 16Ka due to internal factors on their supply transformers
It has been suggested that taking an end of line Zs and deducting the R1 + R2 will give a better indication of the true Ze and then calculate the PFC based on that.
Also remember that the certs. state that Ze and PFC can be obtained by enquiry
If you enquire then you will be given Max 16 KA and 0.35 ze for TNCS
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I would strongly advise you to get somebody else to test it or borrow another meter to test it. I had one of those meters and it packed up in the end telling me I had low readings on a TT system. Luckily it was in warranty so I got a different meter to replace it.
sparkyjohnny
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For those values the transformer would have to right outside, (it is possible to get above 16.5KA even on domestic, but very rare) get a KT 64/65 or Fluke KT 1653 and do the test as a 2 wire and 3 wire (3 wire to earth and 2 wire to Neutral).
Testers normally around 5% accurate when doing IpF testing, so have you been checking yours on a check box/known supply?
All the best
Testers normally around 5% accurate when doing IpF testing, so have you been checking yours on a check box/known supply?
All the best
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