How hot do 45A isolators run under full sustained load?

[Tim Watts]

(Spun off from Chris' thread).

Just noticed my water heater isolator (about 45A load) gets rather warm after 15-30 mins of operation.

Tightened the terminals - but it seems to be the switch element - the toggle base gets to perhaps 40-50C by feel. Never noticed before because I'd never really gone around feeling shower/water isolators under high sustained loads!

Personally I'd expect a simple switch to be stone cold even under permanent full load.

Is this reasonable - or time to replace? It's a GET plate - only 6 years old.

Cheers,

Tim

[ARW]

I would expect the cable could be at 40C at that load.

However if you suspect that the switch is getting hotter than the cable then swap the switch [1] as it is only a few quid and see what happens.

[1] A Yorkshireman can spend other peoples money:-)

[Tim Watts]

Thank you Adam. Did not think to check the cable. IIRC I used 10mm2 for that one.

Thinking P=I^2 x R, even if the switch has a contact resistance of

0.01 Ohms, that, for I=40A, would be 1600 x 0.01 = 16W developed across the contacts. In a fully plastic enclosure, I guess that is actually quite a bit of heat. Could be reasonable I suppose...

Yeah - I'll get a new switch of the same make (for same terminal layout) next time I'm up TLC in a week or two.

[spuorgelgoog]

Maybe Amendment 4 will require all switches > 16A to be in non-combustible enclosures.

Owain

[mick]

It's difficult to be certain. For fairly modern stuff it's "not hot enough to cause damage to the contacts or operating mechanism"! You'd really need to look at the manufacturers data sheet as that will indicate stuff like derating due to ambient temperature.

[Andrew Gabriel]

Yep - datasheet (at least for professional quality parts) will tell you, because the heat from a breaker running at full load reduces the overload trip rating of the adjacent breakers. Hence there is sometimes a requirement to space out breakers expected to run at full load for long periods, so they don't overheat or take themselves or adjacent breakers too far from the rating plate.

[Tim Watts]

Just to clarify, this is an isolator - not the MCB/RCBO (which runs stone cold BTW).

I lied about it being a GET - it's an MK:

Which is odd as all my other stuff is GET. Not much on the datasheet - nothing useful like contact resistance or derating :(

[Dave Liquorice]

Hum, a plain switch running hot when the associated MCB/RCBO is cold. Bear in mind that MCBs have a thermal part...I think I'd be down the shops tommorow or not use what ever it is until the switch has been replaced.

Why is terminal layout so critical? OK 10 mm2 isn't the most bendy stuff but the switch will be on a 47 mm deep box won't it to give room. Take a drawing of the layout with you so you can get something close.

[Tim Watts]

Well, even I'm not going to panic quite that much :) This is only used when people are around.

There's not much slack...

[harry]

If the terminals are tight, change the switch. The contacts must be burnt/defective A shower is only run for a few minutes, there's no way it should get even warm.

[I love spam]

One other thing to think about is to uprate the switch to say a 60A, that will give you more headroom between the actual current flow and the maximum current rating of the switch, that hopefully would reduce I2R losses across the switch and hence reduce the overheating effect?

[Tim Watts]

I'm not aware anyone makes a 60A shower isolator?

[Tricky Dicky]

Crabtree make a 50A one

Richard

[Tricky Dicky]

Lap make a 50A pull switch

Richard

[Tim Watts]

Well - I had a mate round (not a sparks) and I said: "touch that" (we use highly scientific testing methodologies in this household!)

"Bloody hell, that's hot" he said.

At which point I wandered off to B&Q (being open late on a Saturday).

Interesting point - bought a Crabtree 50A unit to replace - the Crabtree has twin screw terminals (except earths) +1. It seems to have a ceramic switch unit mounted to a plastic body +another1 It says: "50A" on the back and the packet.

The MK one (identical to the current one) is in a box on the shelves where the box and the packet say "50A" but it has "45A" printed on the plastic. -10 for lying. The terminals are the same single screw type mine is and the neon still has 2 weedly exposed wires that get pushed around by the 10mm2 going into the terminals.

Conclusion: MK is not the brand it used to be.

I will attempt to disassemble the old one and see if there's any interesting tell tales of overheating inside...

[charles]

Having looked at Wiki to check that I was right about the current owner ship of the company. I noticed that there is a "product recall" of certain switches. Could the one you have be affected?

[Tim Watts]

That's a CU main isolator, so anyone with an MK CU would be well advised to check it. Good mention.

Not my part though - which is a K5215.

So Honeywell own them now... Hmm.

[Dave Liquorice]

But you still did when your mate came round and commented. B-)

Tricky I know to leave slack with heavy cables, shallow boxes don't help.

[Tim Watts]

Yes - sometimes it helps when some else agrees on the spot :) My sense of temperature by feel is not a well calibrated instrument - it helps to have a second opinion.

Yes it is - and I used a 47mm box (of course - I'm not that stupid), but I did not leave much slack. So I will move the new box back a bit and replace the short run to the heater. That'll give me more room to get to the screws at the wall end.

I slapped the thing in a hurry some years ago - none of it was optimal... Will do it a bit better this time.

[Tim Watts]

Well, replaced after some hours with a Crabtree 50A isolator - and after filling a bath, it is virtually stone cold. The rocker has a hint of warmth, but you'd be hard pressed to tell.

So the ultimate answer to the subject is "hardly at all".

Apparently according to a sparky I was talking too (not Adam) have have also been making a mistake of twisting the strands in 10mm2 cable - something my dad always did....