20kA ipf on 15A 3036 fuse

[Aaron b]

Hello, I changed a socket over earlier and tested the circuit after. The fault current was 20kA, Am I right in thinking I only need to consider the breaking capacity of the service fuse that's likely to be 80kA? I'm struggling to find any reference material.

 

[UKMeterman]

20KA, more details needed, can we have details of the supply, photos of the supply meter, cutout and consumer unit. Please measure distances.

 

[Aaron b]

20KA, more details needed, can we have details of the supply, photos of the supply meter, cutout and consumer unit. Please measure distances.

It's a tns supply fed from a pole mounted transformer about 15m away. Lead sheathed cable with a pipe clamp providing the earth. 2.5mm cable feeding the socket about 1.5m from the intake. It's a 60amp supply. Just upgraded to a smart meter. The guy done a nice job and sorted out the mess the PV guys made years back. It's a white plastic wylex board with a wooden back.

 

[Lucien Nunes]

That is an unusually high fault current and MFT results at these levels can be rather inaccurate. The problem is that when the impedance is low, many ordinary instruments can only measure it with low resolution, so when the fault current is calculated from the impedance it can have large hidden errors.

Was the 20kA calculated by the MFT and if so what was the associated impedance measurement? An Ipf of 20kA implies an impedance of 0.0115Ω but even an error of say 0.01Ω due to incorrectly compensated lead resistance would bring the current down to 10.7kA. That might not get your rewireable fuse off the hook, but it's worth being aware of this source of error in Ipf readings.

 

[UKMeterman]

It is higher than the prospective fault current advertised by some DNOs. I would suggest asking the DNO what is the the actual fault current. It may also look like a board change required with regards to the 3036 fuse links.

 

[Aaron b]

I think you are probably right. I just ran a couple of Z's tests on a socket in my property and was getting massive L-N currents. I was zeroing using the plug clip thing but trying a different one brought down the readings to something more reasonable.

 

[pc1966]

That seems astonishingly high for that sort of a setup. The 500kVA transformer that feeds our supply point is only 11kA (three-phase bolted fault, I measured 5kA L-N which would be 10kA 3P fault at that point)!

 

[pc1966]

I was zeroing using the plug clip thing but trying a different one brought down the readings to something more reasonable.

Ah, maybe your MFT takes in to account probe-zeroing and it was last set on different leads?

As @Lucien Nunes pointed out, it only takes a few 0.01 ohms to make a massive difference!

 

[Aaron b]

It is higher than the prospective fault current advertised by some DNOs. I would suggest asking the DNO what is the the actual fault current. It may also look like a board change required with regards to the 3036 fuse links.

The site definitely needs a upgrade and hopefully it was the zeroing thing throwing me out.

 

[Aaron b]

It looks like the TIS clip is reading 0.2ohms higher than a fluke one on the live lead. Unfortunately the meter only flashes up with the calibration values for a second so I'm going to have to keep an eye out for that.

Thanks everyone for your input.

 

[UKMeterman]

I think you are probably right. I just ran a couple of Z's tests on a socket in my property and was getting massive L-N currents. I was zeroing using the plug clip thing but trying a different one brought down the readings to something more reasonable.

Hi,
As you are a trainee I would suggest you ask your local DNO what is their normal advertised PSCC and also disucss https://www.beama.org.uk/asset/1E3E6D54-6091-47E0-805FBA161841A309/ with your supervisor. Eitherway I suspect that 3036 board needs to come out.

 

[pc1966]

Hi,
As you are a trainee I would suggest you ask your local DNO what is their normal advertised PSCC and also discuss https://www.beama.org.uk/asset/1E3E6D54-6091-47E0-805FBA161841A309/ with your supervisor.

That is a useful guidance note to keep on hand.

Either way I suspect that 3036 board needs to come out.

Very true.

While they are still considered "OK" for some existing installations, they have a number of significant drawbacks in terms of no RCD/RCBO options internally, limited breaking capacity (as discussed here), sometimes wooden backs so poor fire rating, and, most importantly, few of the general public are competent to change fuse wire.

Also it was usually the lights that would go, and then you had the added joy of trying to rewire in the dark

Not to mention the unpleasant result if someone fails to heed the instructions about switching off and plugs a rewired fuse back on to a live fault.

 

[UKMeterman]

It looks like the TIS clip is reading 0.2ohms higher than a fluke one on the live lead. Unfortunately the meter only flashes up with the calibration values for a second so I'm going to have to keep an eye out for that.

Thanks everyone for your input.

Discuss

with your customer.

 

[UKMeterman]

The site definitely needs a upgrade and hopefully it was the zeroing thing throwing me out.

Print out and take a copy of https://www.beama.org.uk/static/uploaded/fa3799ac-9e73-4680-a5006401014b16b3.pdf to your next jobs.

 

[Aaron b]

I've been working on industrial sites for over 10 years but I'm going through the NVQ to formalize my qualifications and to be able to get part p registration but clearly I'm a bit green with some of the domestic stuff. Thanks for the idea on calling the dno, it's not really an area I've had to deal with in industrial scenarios.