Electric iron intermittently goes bang

[Steve T]

Hi all,

Was speaking to a family member recently, he told me his iron recently made a bang and flash as he plugged it in to the socket, and could I take a look at it.

I asked what exactly happened, so apparently it's happened twice, the first time it was a smaller bang and flash so he thought it might just be a bit of a surge when plugging in a switched on appliance... but it tripped the B32 breaker on that circuit, however the plug fuse was fine. So he decided to reset the breaker and plug it back in - I did say that probably wasn't the most sensible course of action - but it worked fine for maybe a month he said until recently the same thing happened albeit with a bigger bang and flash, again taking out the B32 breaker, plug fuse still intact.

The circuit is RCD protected, the RCD never tripped, its quite old tho so it's anyone's guess to whether it actually works. Anyway this time he realised there might actually be something wrong so he hasn't used it again.

It appears that the fault current from what is probably a short to earth/neutral has also damaged an electronic fridge thermostat, it still works but now constantly flashes 'Err' on the screen, resetting doesn't fix it. Would you expect a direct short on an appliance to damage other electronic appliances? I'm not sure if the fridge and iron were actually plugged into the same circuit but both circuits are on the RCD side of the board. Or is it coincidence?

I've taken it off him out of curiosity as I'm intrigued what sort of internal fault would cause an intermittent live - earth/neutral short. Are heating elements known to intermittently fail in that way? It's quite an old iron. Other thought I had was maybe if the cable is flexed in a certain way two damaged conductors make contact, but there's no visible damage on the outside. How would you go about testing it? I'm thinking the first test should be insulation resistance, what else would you check? Obviously I won't be plugging it in again but would be interesting to know what's happened.

Cheers for any replies!

 

[timhoward]

If it's not taking out an RCD, then either the RCD is stuck or it's not a fault to earth, i.e. a L-N fault.

It's curious the fuse in the plug isn't blowing. Check it actually has a fuse in the plug not a nail!

Yes, IR test it. Then throw it away!

(I'd keep your mind is open to the small possibility that the fault is with the socket it's plugged into not the appliance itself.)

 

[Steve T]

If it's not taking out an RCD, then either the RCD is stuck or it's not a fault to earth, i.e. a L-N fault.

It's curious the fuse in the plug isn't blowing. Check it actually has a fuse in the plug not a nail!

Yes, IR test it. Then throw it away!

(I'd keep your mind is open to the small possibility that the fault is with the socket it's plugged into not the appliance itself.)

Wouldn't a L-E fault also take out an MCB if the Zs is low enough, this is a TNCS system?

The fuse is 13A, ATLAS branded, of course there are dodgy fuses out there so who knows.

Thats a good point at the end, if I can't find anything wrong with the iron the socket may need looking at. To be fair he's only ever used the iron in the one socket which is where the bang has happened twice, its a metal decorative one so I suppose higher chance of a fault there!

Will let you know the results of IR testing.

 

[pc1966]

Wouldn't a L-E fault also take out an MCB if the Zs is low enough, this is a TNCS system?

Yes it would, but it would also trip the RCD.

So if MCB goes and no RCD then as @timhoward said the RCD must be faulty (check with test button?) or it was L-N.

The fuse is 13A, ATLAS branded, of course there are dodgy fuses out there so who knows.

There is a range of fault currents that will hit the magnetic trip on a MCB but it clears quickly enough so the 13A fuse is OK.

The fuse would be weakened though, and I would be inclined to replace it just in case.

Thats a good point at the end, if I can't find anything wrong with the iron the socket may need looking at. To be fair he's only ever used the iron in the one socket which is where the bang has happened twice, its a metal decorative one so I suppose higher chance of a fault there!

If you can't find fault with the iron then replace the socket, more so if no-name or otherwise suspect. An internal fault there would take out the MCB and fuse would see nothing. But faulty socket would be a rare thing.

Will let you know the results of IR testing.

Good to hear!

 

[Steve T]

Yes it would, but it would also trip the RCD.

So if MCB goes and no RCD then as @timhoward said the RCD must be faulty (check with test button?) or it was L-N.

There is a range of fault currents that will hit the magnetic trip on a MCB but it clears quickly enough so the 13A fuse is OK.

The fuse would be weakened though, and I would be inclined to replace it just in case.

If you can't find fault with the iron then replace the socket, more so if no-name or otherwise suspect. An internal fault there would take out the MCB and fuse would see nothing. But faulty socket would be a rare thing.

Good to hear!

Cheers, so I've done 500v IR testing LN to E on the iron as a whole, its coming back clear at >999, IR at 1000v between all cores of the the cable and moulded plug by itself (disconnected from iron) is >999 all round.

Resistance of the heating element is around the 30 ohm mark, which seems correct for it's 1700-2000w rating. Tried flexing the cable as the tests were running, no change in numbers so I think I can rule out a broken conductor making contact in a certain position, also no black or burn marks anywhere on the cable, plug or inside the openable bits of the iron.

Can an iron heating element go intermittently short circuit? Bearing in mind the one in question still worked for a while after the first event and is now testing fine.

Seems like it may be worth taking a look at the socket...

 

[DPG]

I'd definitely check the socket yes.

 

[Steve T]

Interesting one, it definitely is the socket at fault, looks like a bit of metal must have got in there somehow and shorted L&N, don't know where it came from but it's certainly vaporised now! Wasn't a piece of copper from the cable behind the socket, its solid core T&E. Strange that it took 2 bangs to get rid of it as well! I'm guessing a manufacturing fault leaving a bit of brass sticking around that never got noticed until the socket started to get used more. It's VARILIGHT branded which I hear is decent enough.

I asked for more detail about when exactly the bang happened, apparently both times it was after plugging the iron in and switching the socket switch on. Makes sense looking at the damage.

 

[Steve T]

 

[Steve T]

Also checked RCD trip times etc. are all within limits so its not been fried by 2 L-N shorts, Zs is 0.33.

 

[James]

That's some good forensic, after the fact fault diagnosis.

Well done for finding a tricky intermittent fault that had cleared itself so there were no measurable faults left to find.

 

[littlespark]

One of my wife’s friends asked me to look at an iron a few years ago… kept saying there was a flash when she plugged it in.

No wonder… iron was turned on, as was the socket…. In a fairly dark room.
She was plugging it in full load… you could see the arcing through the white plastic socket.

And of course, she was unplugging it full load as well!

A little instruction was all that was needed.

 

[Steve T]

That's some good forensic, after the fact fault diagnosis.

Well done for finding a tricky intermittent fault that had cleared itself so there were no measurable faults left to find.

Cheers, I was definitely pleased to find it, wouldn't have been happy not knowing the cause of two apparent direct L-N faults, although as you say that socket would actually now have worked fine infinitely!

 

[Marvo]

What is an iron? I feel a bit embarrased asking because everyone else seems to know, just wondered if it's for doing your hair or is it a steam iron for clothes?

 

[pirate]

@Marvo I'm not sure either...why would anyone plug a golf club into an electrical socket?

 

[loz2754]

@Marvo I'm not sure either...why would anyone plug a golf club into an electrical socket?

It's ok if you use the correct driver for the application.