I`m Trying To Find A Rational Explanation.....and Failing.

[kme]

Went to look at a CCTV camera - stopped working - in a public house this morning.

The camera takes a 230V supply, so has a 1361 plugtop on it, with a 3A fuse in.

The single socket it plugs into is on a 0.5mm 3 core flex, approx 10 metres, to a fused spur - which had a 13A fuse in!

The spur is supplied from an adjacent socket outlet - the circuit is derived from a TPN DB, via a "Bill" C20 MCB (3871).

The TPN DB is on a B63 Wylex from the origin.

The 13A fuse in the spur had blown. I tested the extension cable, to the single socket, ins, with Phase and Neutral together, as I couldn`t access the socket (customers in premises) - so checking to earth. >1000Meg @ 1KV. Happy days.

Replaced the fuse with a 3A, and the camera starts working - no fault.

However, it turns out that, when the 13A fuse went, the C20 on the TPN tripped too.........

As did the B63; taking out the whole DB.

So there was a BIG fault occurred on this cable - which then "blew clear", and works fine - but took out everything back down the line.

I may be getting thick in my old age; but I cannot envisage any possible scenario where this could happen! Anyone got any ideas???

 

[ProDave]

You really need to go out of hours, unplug the camera, and IR test L-N

 

[septiclecky]

Some one unplugged the camera and plugged something else in but aren't saying

 

[kme]

Dave - yes, I intend to.

Septic - I did think that - but its high level, pretty much dedicated socket; and STILL doesn`t really explain taking out the whole cascade of OCPDs

 

[ProDave]

Look for the hole in the cable where someone tried to hang something, and then sheepishly pulled the nail back out after it tripped, and then walked off.

 

[SLIPSHOD & SLAPDASH]

Its not that uncommon to blow or trip a series of OPD's due to their own loads. I once took out a DNO 400A main fuse in an office block when i clipped a .50mm cable in a control panel with a 20mm hole saw. It blew the 1A panel fuse also . I know it was me as the lights went out at the same time.

 

[sprocketflup]

Rational explanation? Got to be quantum.

 

[Sharpend]

We had one last night, office building powered down for testing of submains, went too switch on and blew two phases on the isolator (315A fuses) and the other phase blew through the isolator and the main board in the switch room (550A) fuse but took out the 800A fuse at the transformer?

Still, all replaced and retested, no faults before or after, switched on this time, perfect!?

 

[Sharpend]

This was my thoughts, Canoey, although there was talk of a dodgy busbar chamber possibly being suspect, however this has been ruled out.

 

[Sidewinder]

What "normal" load were the upstream protective devices "seeing" at the instant of the fault?

Could they have been at or "near" overload at the time of the fault?

Are they "tired"?

 

[kme]

They may well be "tired" bud

"normal" loading on the sub DB is circa 40A/phase

Old breakers? Definitely.

Plus - mismatched manufacturers - so cascading isn`t guaranteed............

I don`t believe anyone did anything to the circuit at the time - its in the customer side of the bar.........and went early evening - with a bang, apparently.

The supply is, I believe, 250A BS88s - though we don`t have access to `em

 

[Sidewinder]

To get to the bottom of this I believe you would need to have the manufacturers data for all the CPD's and do a full calculation on the disconnections, but, from the existing base load, at each & every point, remembering the bull rush charts in the regs take the worst case scenario, which is the longest disconnection time, i.e. from 0A starting load in the event of a fault.

 

[kme]

S`what I thought - but it won`t tell me what the hell happened, and isn`t there to happen anymore!!!!!