Popping 100A fuses

[ekmdgrf]

A while ago I read a story about how a spark started off on their career path. As a 7 year old, she decided to test by experiment a thought that I'm sure we all had when we were young: does string conduct electricity? The experiment consisted of a couple of chairs, with the sample piece secured between them. Attached to one end of the string was a metal prong, which was inserted into a 13A socket. The experiment had a couple of huge mistakes that rendered the test inconclusive: firstly the makeshift busbar in question connected to both the line and neutral of the socket and, of course, secondly there was no return path from the other end of the string. Further frustration was caused by the initial abortive attempt popping the 100A service fuse and rendering further testing impossible. Apart of the obvious thing that one would think upon reading this: "so *does* string conduct electricity?" two other thoughts crossed my mind:

1) How often do 100A service fuses pop?
2) When you telephone the DNO, do they simply turn up and change it or do they shin a 500W flood light at you and give you the DNO inquisition?

 

[ericmark]

Only DNO can answer fuse question. I have only seen one blow and that was due to using a battery charger converted to turn meter back and he called me as there were black marks on the fuse carrier.

I have I will admit worried about blowing the DNO fuses watching the current approach the 400A mark when fuses were only rated 350A and realised if they went there would be a panic trying to get concrete out of machines before it set.

I am sure the DNO would have asked questions but more about bringing in more power than why they failed. But my question would be why did DNO fuse go one would have expected the B32 MCB to have tripped well before any DNO fuse so if I was the electrician attending I would be concerned.

I have seen where two B32 MCB's have fed the same ring. I have also found what was thought to be a MCB but was in fact nothing more than a switch.

As to wet string conducting this is a hard question. There have been cases of wet guy ropes after lighting strike being fried which would point to them conducting however how does one tell if conduction or electrolysis?

My son also decided to have electric as hobby then in end became an electrical engineer so as a caring dad one of my early jobs was to fit RCD protection and yes he did trip them from time to time.

We do tend to jump to wrong conclusions like Wellingtons which have graphite added to the rubber mix to stop problems with static discharge so they do conduct electric the same applies with car tires may be high resistance but they do conduct. Hence why they melt when power lines drop on them. However they were not always that way always told if trolley bus was to crash always jump off do not step off.

 

[NewOrder]

About ten years ago when I was younger my dad drilled through a cable. I was too busy with new mobile phone to play any attention to what he was doing. It didn't blow the fuse at the consumer unit but it did blow the DNO fuse.

I know instantly what had happened as all the circuits in the house were dead. DNO came out, I had to convince him that I had isolated the damaged circuit and he re-energised it.

Still not sure why the fuse at the CU didn't blow but the main DNO one did.

He has never made the same mistake again though! I am still a bit concerned to why the fuse at the CU didn't blow, but when there was a fault on the light circuit the fuse did blow then.

 

[westie101]

In the first instance we would try to work out why the fuse had operated and carefully inspect around the meter position.
As a precaution turning any CUs off before replacing the fuse.

More often than not it is due to load or a poor contact in a 60A cut-out (which we will arrange to replace).

If further events occur we would monitor the load for a week and if constantly overloading would deal with that appropriately

 

[ricicle]

I blew one when I was an apprentice. I cut through the wrong cable and the circuit was protected by a 6" nail in a wylex fuse.
I've also blown a 500A BS88 in our factory but that's another story....

 

[kai]

Has one ever encountered the case where the Service fuse remained intact, but the substation fuse upstream operated instead?

 

[OwainDIYer]

Has one ever encountered the case where the Service fuse remained intact, but the substation fuse upstream operated instead?

Not yet.

 

[undistilled]

Has one ever encountered the case where the Service fuse remained intact, but the substation fuse upstream operated instead?

That might be easer if you had access to decent tools, like a JCB.

 

[westie101]

Has one ever encountered the case where the Service fuse remained intact, but the substation fuse upstream operated instead

Once or twice where the transformer is in the 15 to 25 kVA range with a 100A fuse

 

[bernardgreen]

My biggest was dropping a pair of pliers onto naked 3 phase bus bars in a ships power cabinet. It was on a shore supply fused at 200 amps. Quite a firework display before the fuses went, and it ruined the pliers.

I got a lot of aggression from the workers who were plunged into darkness. The alternative, safe working was to turn off the power to the whole ship before I started working in the cabinet but that would have halted work for several hours.

 

[flyingsparks]

Chap I use to work with was fitting a new earth from the met on the cut out to the new fuseboard he had just changed. Met connection done, he accidently let go the earth wire which sprang back and ended up going into a live Henley block where an extra knockout had been opened and not sealed up again.

You could have do e that 100 times and never had it happen again I expect. As far as i am aware the DNO just came and replaced fuse no questions asked.