Fault Current Too High

[RussellR]

Hi All

Wondered if anyone had any opinions on this one, I did a Periodic yesterday on a smallish unit, which was part of a whole estate, it had a three phase 100 amp supply split off via henley blocks into one wylex (addition) and on merlin Gerin (original installation) dis boards. ZE at both baords was coming up as 0.01ohms giving me a PEFC of 19.8 KA and PSCC Measured from Line to Neutral of 19.9 KA, Doubled because they are both three phase gets me up to nearly 40Ka.

The breaking Current of the MCB's is 10Ka and 9Ka for the merlin stuff and 6KA for the Wylex. I am puzzled that the instalation is 18 years old and had numerous additions, that in fairness are of a high quality, but this has never been flagged. Also what would you guys offer as a cost effective solution.

Russell

 

[Andy™]

the BS1361 or 88 will give you your high PFC protection

 

[RussellR]

Thanks Andy

Is what you are saying that the service fuse would limit the current to acceptable levels ?

Now i am confused.

The average domestic breaker is rated at 6Ka which is 6000 amps, so that would mean on a supply which was less than 6000 amp we wouldn't need to worry about the breaking capacity at all ?

Or am I thinking about this the wrong way.

Thanks for your help

Russell

 

[Andy™]

there needs to be at least 1 device which can break the PFC. normally the MCB, but in some cases, the service fuse is backup. the service fuse will NOT limit the fault current. in the unlikely event there is a fault with a PFC of 19.8KA and 6KA breakers, the breakers may not be able to break the current and need replaced, but the service fuse will break the current

 

[RussellR]

Thanks Andy, but in an extreme case couldn't the breaker explode or catch fire, even with limited exposure to the fault current ?

 

[batty]

That seems a very high fault current have you had your meter checked lately.

Batty

 

[RussellR]

Hi Batty

Yes Checked it on two different meters, both of which have been recetly calibrated, and are checked against a check box each month.

The substation is very close by.

 

[batty]

When I am not sure about something I ask my part p assessor he normally comes up with a solution to most things. I have my assessment on Wednesday I will ask him what he thinks if I remember.

Batty

 

[RussellR]

Thanks Batty

I think Andy is correct. He usually is.

I don't have my regs with me but found another post that quotes so will check that out when I get in, in the morning.

Reg. 434-03-01

A lower breaking capacity is permitted if another protective device or devices having the necessary breaking capacity is installed on the supply side. In this situation the characteristics of the devices shall be co-ordinated so that the energy let-through of these devices does not exceed that which can be withstood, without damage, by the device or devices on the load side.

Thanks Guys really appreciate your input.

 

[batty]

I would not be happy with this situation. I think I would look at changing MCB's for cartridge fuses or at least advise this on report. I have never come across this situation apart from having rewireables that have needed replacing with MCB's.

Batty

 

[wozz]

would the DNO be interested in this situation?

 

[bod71]

I have had this high reading a couple of times.

Not a great deal you can do about it, it is what it is.

In my case, I was working right next to the sub station which

accounted for the high pfc.

 

[septiclecky]

would the DNO be interested in this situation?

Why?

Such a high fault current can depend upon how close to the transformer the installation is.

 

[Andy™]

Thanks Andy, but in an extreme case couldn't the breaker explode or catch fire, even with limited exposure to the fault current ?

MCB may possibly fail, but if you have such a high fault current, then using time/current graph you can find out just how quickly the main fuse will open, so overall let through is unlikely to cause a fire at an MCB

 

[Andy™]

I would not be happy with this situation. I think I would look at changing MCB's for cartridge fuses or at least advise this on report. I have never come across this situation apart from having rewireables that have needed replacing with MCB's.Batty

why go to the hassle of something un-necessary? in reality, fault currents will be much less, since its unlikely to be inside the CU or very close. but im not saying that the full fault current will never happen, since it could

 

[wozz]

Why? Such a high fault current can depend upon how close to the transformer the installation is.

i have seen reading such as this in units very close to the transformer. But i was of the understanding DNOs have resistors at the transformer to stop the Ze falling very low thus very high fault currents? or is that complete carp?

 

[Riggy]

It strikes me that if you had a 20kA fault, whether a Wylex MCB was still operable might be the least of you worries.

If the service fuse doesn't clear the fault the molten cable will.

 

[Andy™]

i have seen reading such as this in units very close to the transformer. But i was of the understanding DNOs have resistors at the transformer to stop the Ze falling very low thus very high fault currents? or is that complete carp?

almost complete crap.

there may be a high impedance connection to earth (IT supply), but this would be under controlled conditions, and never happen to a standard DNO supply