High PFC

[andyb]

I’ve just been to look at a job where the client wants to have a couple of extra sockets fitted.
The consumer unit is an old rewirable with no rcd protection, but there is a new garage rcd/mcb board fitted next to it with a spare way that was installed just 2 weeks ago by their landlords electrician when he installed an electric heater.
Ok, should be easy, just test the existing circuit that I’m going to extend and put it into the new board on a 32 amp mcb.
The electrician only installed 16mm tails and 10mm earth but I can easily up grade these, the problem I have is that I did a quick Ze and got a very low reading of 0.04 ohms. This prompted me to do PFC test and I got a result of 11Ka.
Now, the new garage has domestic 6K breakers fitted, I know I can get 10K mcbs but I do not know of anything with a greater breaking capacity.
Any ideas ?

 

[Mr Mark Sparks]

the 11ka is the reading you would get if a fault occured before the mcb if / when you do an eli at your new points or the exsisting ones it will be far lower than that

 

[andyb]

The 11 Ka is the maximum current that could flow in fault condition.
Therefore the breaking capacity of the mcb has to able to cope with this, this is why you note it on the certificate.
If the pfc was say 7Ka I would fit a 10Ka mcb, but as it's 11Ka even this would be inadequate.

 

[DanBrown]

I know this might be besides point now, but according to the OSG if you have a 11KA PFC, a 10KA breaker would be adequate, but would only be able to interupt a single fault before the breaker would be unusable. A 10KA would be able to keep in performance upto a 10KA fault, but after 10KA to 20KA should be able to perform once only.
Would like some confirmation on this, as this is something new ive learnt.. or i think lol..

 

[Mr Mark Sparks]

you wont get.04 or 11ka after the mcb your .04 reading is at the cu further down the installation it will drop quickly,
eli an outlet fed off that cu and see what you get

 

[andyb]

The rated short circuit current must be less than the pfc at the point where it is installed. OSG Pg 56.
The device is installed in the board, the pfc at the board is 11Ka.

 

[Mr Mark Sparks]

yes but if there is a fault at the board the mcb is not protecting the board nor carrying the fault the device that protects the board (main cut outs) will do that and need to be 11ka or more if the fault is after the board the mcb will cover it but the pfc wont still be 11ka by this point

can anybody else confirm this is so.

 

[Spudnik]

Now chaps, i could be wrong but i calculate PFC at around 6ka?

 

[Guest123]

Hey.

In relation to your Ze, your PFC should be around 5.8KA.

Your reading is a bit high, is the installation right next to a transformer/sub-station??

 

[Spudnik]

I know thats probably the reason the Ze is low which, in turn, increases PFC but 11ka is nearly double what it should be!

Possible faulty tester or leads??

Would be interested to see the readings for PEFC or PSCC.

 

[Guest123]

I concur.

 

[Mr Mark Sparks]

yeah 230/.04 5750 lets assume it is 11ka do you get what im trying to say and if so am i correct

 

[andyb]

The house is close to the sub station,
The Ze was what made me do a pfc test because it is a t-n-s supply and the earth is just a bit of 10mm connected to a clamp on the lead sheath, (I think the last electrician fitted this, it's not right but i'm not going to touch it)
I did the pfc test across live & neutral.

 

[Guest123]

Hey Mark, I was trying to find the reg for you - 434.5.1, P76, I believe is the one.

 

[andyb]

Thanks Lenny.
Back to my op, what can I do about it? and does anyone know of an mcb with a higher breaking capacity?

 

[Spudnik]

Andy,

Your PFC does sound very high indeed and calculation gives nearer 6ka.

Would your meter be giving a false reading?

I dont think there are any MCB's rated over 10ka, unless there may be some specialised manufacturers.

 

[Guest123]

I know of no MCB above 10KA. Do you have a Fluke multifunction tester?? I've heard on the grapevine that these particular testers do have issues with fault current measurements.

 

[andyb]

Andy,

Your PFC does sound very high indeed and calculation gives nearer 6ka.

Would your meter be giving a false reading?

I dont think there are any MCB's rated over 10ka, unless there may be some specialised manufacturers.

Jason, what calculation have you used to get 6K?

 

[Spudnik]

Andy,

Ohms Law.

I=V/R

 

[Guest123]

230/0.04 = 5750.

 

[andyb]

Andy,

Ohms Law.

I=V/R

Thanks for the lesson in electrical theory guys but as I said in an earlier post I got the pfc across live & neutral and I have not posted the live neutral loop reading.
I think I will have to use cartridge fuses and not mcbs.

 

[Guest123]

Well where does your 11KA come from then???

 

[andyb]

My 11Ka came from my tester, I will go back tomorrow and do a line neutral test.
The pfc is definately 11Ka, my meter is calibrated and the reading is correct.
My op was about how to overcome this, and as I said in an earlier post it probably just needs a cartridge fuse.

 

[Spudnik]

My 11Ka came from my tester, I will go back tomorrow and do a line neutral test.
The pfc is definately 11Ka, my meter is calibrated and the reading is correct.
My op was about how to overcome this, and as I said in an earlier post it probably just needs a cartridge fuse.

It doesnt matter, as you record the highest of the 2 anyway. I would hope the PSCC is not higher!

All we are saying is that, by calculation, the reading you are getting from your meter seems remarkably high, which is very concerning.

Let us know how you get on tomorrow!

 

[andyb]

It doesnt matter, as you record the highest of the 2 anyway. I would hope the PSCC is not higher!

All we are saying is that, by calculation, the reading you are getting from your meter seems remarkably high, which is very concerning.

Let us know how you get on tomorrow!

11Ka is my pscc, across line & neutral. As I've been saying all allong.

 

[Spudnik]

Fair enough.

Im sure i have read somewhere that its ok to have a higher PFC than the breaking capacity of the CPD.

Reg 434.5.1.

 

[Guest123]

Hey Mark, I was trying to find the reg for you - 434.5.1, P76, I believe is the one.

Same as me then.

 

[andyb]

Yes I read that.

The first paragraph tells you that the rated short circuit breaking capacity of each device shall not be less than the maximum pfc at the point at which the device is installed. Easy.

The second paragraph then goes on to say that a lower breaking capacity is permited if another protective device having the necessary rated short circuit breaking capacity is installed on the supply side.

I'm not sure I understand this, if it means that a bs88 main fuse with a breaking capacity of 80Ka makes it ok, then there is never any need to worry about pfc. Or have I misunderstood the paragraph?

Had a quick look today, the L-N loop is .02 ohms, the supply voltage is 250v (another issue)

Looking in the osg page 56, the final paragraph seems to say that consumer units complying with BS EN 60439-3 are ok for fault currents up to 16Ka.

 

[kkc35348]

Why can't a BS88 switch fuse be installed before the consumer unit in this case?
Its breaking capacity of around 80kA will safely clear any fault, as a backup to the MCB's.

 

[scotsparky]

The house is close to the sub station,
The Ze was what made me do a pfc test because it is a t-n-s supply and the earth is just a bit of 10mm connected to a clamp on the lead sheath, (I think the last electrician fitted this, it's not right but i'm not going to touch it)
I did the pfc test across live & neutral.

Hold on you said you did a PFC between Live and Neutral??? PFC is between Live and Earth You have measured a PSC which will be different on a TN S system. try it again on between Phase and Earth

 

[Guest123]

Hold on you said you did a PFC between Live and Neutral??? PFC is between Live and Earth You have measured a PSC which will be different on a TN S system. try it again on between Phase and Earth

Hang on chaps, we seem to be suffering from a difference in terminology.

PSCC = L-N fault current measurement
PEFC - L-E fault current measurement
PFC - is the highest of the two above readings noted on the test cert.

 

[scotsparky]

Lenny i just remember it as

PSC Prospectice shortcircuit current L-N
PFC Prospective Fault Current L-E

never used PESC

but agreed biggest on sheet.

 

[old dog]

Hang on chaps, we seem to be suffering from a difference in terminology.

PSCC = L-N fault current measurement
PEFC - L-E fault current measurement
PFC - is the highest of the two above readings noted on the test cert.

yes lenny me and GN3 make you right

 

[andyb]

Hold on you said you did a PFC between Live and Neutral??? PFC is between Live and Earth You have measured a PSC which will be different on a TN S system. try it again on between Phase and Earth

The maxium prospective fault current is the current that would flow in the event of a fault either between live & earth (maxium prospective earth fault currrent) or between live conductors (maximum prospective short circuit current), whichever is the greater.

 

[Notsosmart]

Had a quick look today, the L-N loop is .02 ohms, the supply voltage is 250v (another issue)

Andy,

You will measure 250v as this is the open circuit voltage across the sub-station windings esp as it sounds like you are sitting on the transformer! lol
It may be worth phoning the manufacturer of the CU; if they build to BSEN 60439-3 then this makes allowance up to 16k using 10k mcb's ~ speak to their tech dept. Remember IEE regs defer to manufacturers instructions.

I hope this is useful ;0)
Gaz.

 

[andyb]

Thanks Gaz,
if you look at my post of 3 weeks ago where you lifted my quote from you will see that I mention that.