Fusebox burn out.

[Theo]

I suppose there might come a time when you need to connect an external cable run to an internal cable run, and an isolator seems like a useful thing to have. I'm half tempted to put one in in advance of getting an EV, as it means I can install a tidy internal cable run when doing other works. Roughly like a FCU but with a bigger switch (and no fuse). (and then go through the wall to a SWA termination box on the other side)

Something like:

wouldn't look like too much of a bodge? (LGA Selectric have a similar one)

Possibly ethernet backhaul for a smart charger?

Yep, that's what my money is on.

Theo

[Steve Walker]

Unlikely. My CU can be fitted with split busbars (of varying length) for RCD use or a single busbar for non-RCD use/RCBOs. The size of the busbar is the same in all cases - as indeed it is for the larger boxes with more ways. If split busbars are used, the second is fed, via the RCD, from the first, whcih has to carry all the load on the unit.

That makes sense.

[Steve Walker]

Even if they were, I presume (like the ones I used to specify at work), that they are still capable of tripping if the lever cannot move.

[Steve Walker]

Just looking at the discolouration, I'd say the the terminal on the RCD was definitely loose. The others may have been or it may have been heat transferred along the busbar, from the RCD terminal.

[Fredxx]

I bow to your greater knowledge :-)

[Martin Brown]

It looks like it failed first though and then cooked the distribution panel. The damage to it is way too much for it to be an innocent bystander. It was most likely the root cause and drew enough current to damage other components but not quite enough to trigger a fast cutout.

One terminal is essentially completely burnt away. That is *really* hot!

[Roger Hayter]

Could the busbar be the wrong side of the terminal in the RCD, or can't you do that with modern kit?

[Fredxx]

Both defects looked as if they occurred simultaneously, the switch was the first to let go.

I don't believe the heat travelled up the cable to the consumer unit, or that the switch caused an increased load?

At a guess the charring temperature of plastic is quite low, ~200C when carbonisation starts to occur.

[Theo]

I'm not sure I quite buy that. If everything was making good connections, you would need:

1. The isolator to fail short circuit between line and neutral. That would seem to be a really bad failure mode for an isolator, but OK let's assume it did
2. It fails taking >>32A but less than the trip current/time of the MCB.
3. That current is sufficient to cook the busbar in a short enough time before the MCB tripped (eg 4x rated current it should trip in 10 seconds if I read the graph right)

The MCB is rated for a short circuit current of 6kA so I would be surprised for it to cook rather than simply open and continue with its day. Similarly the RCD should be able to pass 80A with no heating.

If, however, the connections at both ends were high(er) resistance by being incorrectly installed, that would cause heating of both the isolator and the busbar. That could happen in normal operation, a condition which wouldn't trip anything as it's all under the rated current.

Theo

[The Natural Philosopher]

a 45A cooker switch wont necessarily take all the cooker elements on and heating up, for 6 hours...every day.

[Andy Burns]

Even if fitted in a commercial kitchen? Yes I suspect such kitchens would typically have 3ph ovens and would have more chunky isolators, but where does a 45A isolator say "not for continuous use"?

Surprisingly BSEN 60669-1:2000 doesn't appear to have anything to say on that.

[Tim+]

One feels it ought to. A 13amp plug & socket might well be fine for a while, but if you try to draw 13amps for long periods you soon find out that they overheat. EV makers are well aware of this and their ?granny leads? for use in standard sockets are limited to 2.2kW.

Tim

[Adrian Caspersz]

Here is another fusebox, burnt in a similar place. (left image in this frozen youtube clip).

When installing these things onto busbars, ye can either fail to correctly tighten them, or completely misalign the busbar 'tang' so that it friction hits the outside body of the terminal rather than the cavity inside, the tightened screw gripping onto nothing.

[GB]

If the electrician was a bit timid about tightening the screws on the consumer unit, you'd think he was probably equally timid about tightening them on the switch, too. They are both carrying the same current. So, I really can't see any reason to look for anything more complicated as an explanation. :)

I'm interested to know whether electricians routinely use torque screwdrivers, or just rely on experience?

[Andy Burns]

Judging by those who put their jobs on youtube, it's now quite common, and CUs etc include required torque settings.

Whether the old hands bother or "know better" ...

[Theo]

I would expect the isolator to be rated for continuous use. However the electrician, who may have done a lot of cooker and shower circuits in the past but be new to EVs, may have got away with insufficiently torqued screws since cookers and showers don't draw currents for long periods.

(even in a commercial kitchen, you might draw max current when cold but once the ovens/rings are up to temperature the load will reduce, and the likelihood of all the rings and ovens running from cold at the same time is small)

Theo

[Fredxx]

They're not cheap.

Given the way motor mechanics now regularly use torque wrenches on anything critical I suspect there will be creep towards electricians using them.

As I said earlier, if you are seen to use one regularly then it does reduce some aspects of liability and litigation.

It also helps calibrate the wrist, I for one have no idea how much I tighten screws. I tend to nip up the screw on the cable and end at some point when I feel failure is likely or where the blade will slip out of the screw.

[Andy Burns]

Should they buy their own torque analyzer, or send their screwdrivers off for annual calibration checking like they do their meters?

[Adrian Caspersz]

You need the BMW tightness test.

#1. Hand tight #2. BMW torque tight #3. broken

If in doubt, tighten to #3 and then back it off a quarter turn ...

FWIW I've seen torque settings specified for screws inside mobile phones and laptops.

[Steve Walker]

Overly complicated when all you need is a bar with a hole and a screw to clamp to the screwdriver and a mass that can be positioned anywhere along the bar and you can check calibration at as many points as you like.

Simple calculation for the effect of the bar itself and then add the torque due to the mass at whatever distance from the screwdriver.