Urgent help with discrimination

[SimpleSimon]

Hi Guys

Urgently need clarification on something please.

I'm currently sitting in darkness in a shop where for the last 24 hours two phases out of three on the incoming service cable have been blowing within 5 minutes of power being restored. So far I THINK I've narrowed it down to a till point. But heres my confusing situation.

TWO phases blowing- All circuits are single phased equipment EXCEPT an air con unit which was isolated first time the fuses blew by the MCB. Since than it has blown several more times.

So looking at it what would determine two phases blowing at the same time with all single phase equipment in place?? Well the landlord engineers have IR'd the phases to earth and they tell me it's all clear and no problem there.
Next we have a data cabinet with two shop floor till points and a UPS system involved. Now interestingly enough the two till points are spread over the two faulty phases with the data cabinet also being on one of the phases. We've taken a guess at thinking there is a fault with the UPS system as this is what looks like the only thing connecting the till points across two phases. So, we disconnect the UPS entirely and switch on one till and the data cabinet. After a couple of minutes the fuse goes again. So this time I only turn on one till point and leave the data cabinet and other till isolated. Fuse replaced and than again it goes. This was the last time. Now I've unplugged all components from both till point circuits and I'm going to run some tests on the circuits.

My question is this- why is a BS88 63a fuse blowing before C16 MCB? Do BS-88 have higher breaking capacity?

Im going to get my book and study the time curves whilist I wait but I was hoping someone would also be able to give me a quick summary of the reasons as I'm afraid time is against me to keep re-reading the pint s in my book.

Thanks

Simon

 

[SJD]

Well the landlord engineers have IR'd the phases to earth and they tell me it's all clear and no problem there.

Did said engineers IR each phase to each other as well?

 

[SimpleSimon]

Yes phase to phase and each phase to earth.

Im thinking a supply fault BUT the only thing that points away from this is when I turned the power on and had isolated 3 circuits Till 1 Till 2 & data cabinet the rest of the shop stayed on for a lot longer than before. It than didn't go off again until I turned on Data cabinet (minus UPS removed) and till 1.

 

[SJD]

Umm, OK re IR tests.

Re BS88 63A & C16 MCB, I'd say the C16 should easily trip first - for example, if >160A fault current then without delay, whereas BS88 would take >10s at that current. Perhaps the C16 breaker(s) are faulty, e.g. having been reset too many times before someone got called out, have you any spare MCBs to check?

 

[SimpleSimon]

Sadly I haven't but I will be looking st doing this ASAP even downrating it to a B16. I agree with you looking at time curves, for an instant trip out this should be the C16 but it's running for a couple of minutes and than tripping, still think its a MCB fault? Could there be some sort of surge from a failing component that is causing a large trip out current?

Simon

 

[Silly Sausage]

Time to get your current clamps out (preferably three of them!) and see what current is flowing in those supply tails.
More information always helps.

 

[SJD]

Yes, are there significant loads connected that mean the BS88 fuses are already carrying a lot of current, and it doesn't take much more to trip them? Or are you testing with everything else disconnected? With no loads, I'd expect the C16 to trip first everytime, even with a surge as something in the data cabinet fails.

 

[Geordie Spark]

Help required with discrimination??

Piece of cake: Whites at the front of the bus, all the rest at the back. :53:

 

[snowhead]

Is that DNO fuses or Landlords fuses from His supply that are blowing.

What's the voltage on the incomers with no load on in the shop?

 

[UNG]

What checks have been done on the incoming supply if you are saying that no faults have been found on the installation then the fault must be with the supply

Have you checked for a floating neutral

Are any other premises in the immediate area experiencing problems with their electrics

 

[SimpleSimon]

Ok
so returning to this thread as I never finished the conclusion (got a bad habit of this. See previous threads)

Turns out there WAS a fault on the supply cable despite engineers telling me they had tested and checked it.

What we did was temporarily move all circuits that where important in getting shop opeingin(tills & servers) onto the working phase and ran the system off a single phase so the shop could trade.

One of my guys than went in at night to support the shop and an engineer from the landlords side came in too. This guy however, checked his cable and found a fault within a hidden isolator switch. Turns out two phases where loose and where arching across.
The guy locked off the two phases and got the isolator replaced as it has started to burn out, and stripped the cable back to a suitable state. Since than,all good.
So there was a problem with the service cable despite me being told "no all tests are c ng back fine but your breakers aren't tripping and our fuse keeps blowing!" I wish I could have tested myself.

Simon

 

[ayjay]

i normally do the "sniff test"
if a connection is loose /arcing , it smells very strongly of burning

 

[markc123]

i normally do the "sniff test"
if a connection is loose /arcing , it smells very strongly of burning

i prefer the "taste test"

if a connection is loose /arcing , it tastes very strongly of bbq ribs