A First In My Career

[securespark]

I came across one of those rare beasts today, a situation I have never come across before!

I came to a job where the service fuse was continually blowing.

UU replaced it with a 60A 1361 fuse, which later blew, as did two further ones.

After much searching, I discovered a disused cooker feed (still connected at the board) which was intermittently down.

But why no discrimination?

The 60A 1361 fuse was blowing instead of the 30A 3036.

Now, I have seen situations before where a B32 breaker will trip instead of a 13A 1362 in a plugtop.

But never a 60A 1361 instead of a 30A 3036!


Has anybody else seen this?

 

[Spark123]

What was in the fuse holder?

 

[securespark]

Standard 30A fusewire.

 

[ban-all-sheds]

Shouldn't be possible, according to the curves, even with the 1361 already under considerable load from other circuits and the cooker circuit fault current only being a 100A or so....

I suppose it was 30A wire in the 3036? [EDIT]Doh![/EDIT]

 

[Spark123]

I'd have expected just the 3036 to pop, though it does depend on the fault level and the energy let through of the 3036 and the pre-arc energy value of the 1361. Maybe even both pop, but is odd that only the 60A one popped.
I can't imagine the pre-arc energy value of a normal piece of 30A fuse wire being higher than the pre-arc energy value of a 60A fuse, does anyone have figures?

 

[securespark]

Can't help but agree with what you've both written. I'm a bit baffled, hence my thread.

Just trying to understand how it could happen.

 

[nozspark]

Isn't the breaking capacity of a 3036 1.5kA?? and a 60 1361 33kA??

so if the fault current was >1.5kA then the 3036 might not have operated... just looked up breaking capacity and it says that a fuse might not operate if it's breaking capacity is exceeded!

 

[Monkeh]

Isn't the breaking capacity of a 3036 1.5kA?? and a 60 1361 33kA??

so if the fault current was >1.5kA then the 3036 might not have operated... just looked up breaking capacity and it says that a fuse might not operate if it's breaking capacity is exceeded!

Wouldn't that just mean an arc would be created? Surely the fuse wire would have vaporised with that sort of current.

 

[plugwash]

My understanding is if you exceed the breaking capacity you will most likely vaporise the fusewire but the current may keep flowing as an arc through a mixture of ionised air and vaporised fusewire.

I'd bet on the wire not being proper 30A fusewire as the cause. Short of melting the wire under tempreature controlled conditions and comparing with manufacturers specs (which I have failed to find) or maybe deliberately blowing a section with a current controlled power supply I don't see any real way to determine that fusewire really is fusewire and not some other form of wire of a similar size.

 

[Lectrician]

A little while ago I had similar - A fault on a socket circuit caused the main fuse to blow. I probably posted it somewhere - It was a child who had decided to pee in the corner of his room, right over a socket. I guess pee is a far better conductor than water

Also had a call out to a house once. The home owner just said to me that the RCD needed replacing, and the DNO had told her this.

When I got to site the main RCD was off. With only half the story, I assumed maybe the RCD would not hold on due to a fault somewhere, or maybe the RCD really was defective.

First instinct was to turn the RCD on. Followed by a huge bang, and the main fuse blown.

Turned out the RCD was causing a short when turned on. It had already blown the main fuse, the DNO had replaced the fuse, turned the RCD on, and realised the problem. So they then livened everything back up and just left saying the RCD needed replacing!

 

[NotHimAgain]

Determining if two devices will discriminate under fault conditions is complex. As Spark123 has said - a study of pre-arcing times and let-through-energy is required, and even this does not give a guaranteed result over the range of possible fault or overload conditions.

You cannot use the graphs in BS 7671 Appendix 3 to assess discrimination. These just show the maximum values for each data point. Protective devices will operate within a specified range not a specific value.

To compare two devices using current / time graphs you need the maximum data point for the smaller device and the minimum data point for the larger.

Example
Taking the points for 1 second and 0.1 second for the two fuses concerned:

BS 3036 30A
This should operate at 150A (maximum) in 1 second

BS 1361 60A
This should operate within the range 275A to 490A in 1 second


BS 3036 30A
This should operate at 450A (maximum) in 0.1 second

BS 1361 60A
This should operate within the range 450A to 890A in 0.1 second

So, just using this simplistic comparison, if you happen to have a ‘slow’ BS 3036 fuse and a ‘fast’ BS 1361 you have no guarantee of discrimination at fault currents >= 450A.

 

[timtheenchanter]

First instinct was to turn the RCD on. Followed by a huge bang, and the main fuse blown.

if that had been me, the RCD wouldnt have been the only thing needing replacing