Electrical cooker isolation switch

[BillDawgy]

Hi there, first off I'd like to say I'm not an electrician but have very limited knowledge. We've an electrician coming but I have a query that i'd like some advice on.

Anyways I have an electrical ceramic top cooker. Yesterday whilst cooking, it started to cut out and we noticed the isolation switch was nearing too hot to touch. We left it cool down and have come back to it today. We turned the cooker on low in order to find the breaker in the fuse box and turned it off. We tried a few before we eventually found the one specifically for the cooker, We then removed the wall socket and the neutral wire from the fuse box has melted into the terminal. After some research I can see this isn't uncommon and could be an easy enough fix with a new isolator switch with re wiring/insulating but was put off doing it myself which brings me to my query.

Even with the fuse turned off, the isolator socket was still receiving power, not enough to turn the oven on or the radio in the same socket, but enough to make the LED on the electric testing screw driver come on VERY brightly, not just a trickle. We figured maybe some electricity was still lingering in the circuit so we left it for 30 mins. Tested with the screwdriver again and still showed power just as bright before. Returned to the breaker box and turned the master breaker off (all the power to the home), went back to the socket and it had no electrical presence...

How can it still be getting electricity when its breaker is set to off?

I understand the principles of the fuses operating on "rings" and I understand the cooker is one of its own, so this has me puzzled?!?!?!

As I said we've got an electrician coming out but i'm afraid he'll just think replacing the switch is all fine and dandy when really it looks like an electrical fault of somesort to me which has melted that wire (too much power???)

Has anyone come across anything like this? Pictures to follow

Thanks for reading, hope you can help

 

[333rocky333]

Try reading up about neon screwdrivers, they are unreliable, there was proberly still slight voltage possibly in the neutral or induced via other nearby cables, a majority of times these voltages are very low, but will show up on the screwdriver.
To carry out any electric work safely you need an aproved tester

 

[bernardgreen]

Neon screwdrivers indicate the voltage difference between the person holding it and the item the probe is touching is greater than 100 volts ( aproximately ).

They can indicate voltage present when they are touching an earthed item or wire.

If it indicates a voltage when touching an item that is earthed then the person holding it has a potential on their body. This can happen if the person is standing close to cable that has live wires in it. Capacitive coupling between the live wires and the person induces a voltage into the person's body.

Neon screwdrivers are not accurate and in some circumstance can fail to detect a dangerous voltage.

Induced voltages can be very high but due to the very high impedance of the capacitive coupling that is creating the voltage any current taken reduces the voltage to almost zero.

 

[chivers67]

I agree with Rocky those Neons aren't reliable and in some situations can be very dangerous - only use it as a screw driver and nothing else. The switch getting hot is probably down to a loose connection (i'd bet money it's a neutral). Even though you say your MCB is switched off the neutral is still connected

 

[BillDawgy]

some pictures to attach

IMG_20160326_1119063_rewind

• BillDawgy
• 26 Mar 2016
• 1

wiring behind cooker isolation switch, melted neutral coming from fuse box

Burnt out neutral coming from fusebox

IMG_20160326_1137334_rewind

• BillDawgy
• 26 Mar 2016

main switch first one in, isolated cooker switch 4th one from left

With the master down is the only way to remove all power leading to the cooker.


Thanks for your replies! I've tested the screwdriver on many appliances and the cooker isolation switch is the only one to show any electrical power whilst its breaker is off, its just weird and im wondering if it caused the fault in the first place

As I said we're awaiting a visit from the electrician

 

[EFLImpudence]

Thanks for your replies! I've tested the screwdriver on many appliances and the cooker isolation switch is the only one to show any electrical power whilst its breaker is off, its just weird and im wondering if it caused the fault in the first place.

No. As said - loose connection.

 

[ericmark]

I love my neon screwdriver it has saved me when disconnecting a neutral which it turned out was shared. However they only really alert one that something may be wrong. Once alerted then one needs other test gear to find out what has happened.

You are correct it should not really light so it would seem there is likely a fault. But as to what sorry you need proper test equipment.

It starts with the house supply, we have TN and TT with the latter pulling a fuse or switching a MCB does not really isolate. With a modern house with RCD protection on all circuits you have likely three points of isolation the main switch, and two RCD's. With some houses using special RCBO's (that's a RCD and MCB combined) the individual circuits can some times be isolated, but in the main the MCB or RCBO only switches one live supply the line, the other live supply called the neutral is not switched with a standard domestic MCB or RCBO. With a TN supply it does not really matter the neutral is unlikely to raise above other bonded items. But with a TT system (uses an earth rod) it is permitted to vary as much as 50 volt and with a fault it could be more.

What I am saying all houses are not the same. And there is a tendency to jump to conclusions without knowing all the facts. The melted wire is most likely caused by a bad connection rather than a overload so likely the electrician can cure it by just changing the switch/isolator. But when he is there ask him why your neon lights. It may be that you do have a problem nothing to do with the cooker which without the prompt he could miss. But with the special meters we use like the loop impedance tester for the electrician it's a fast test. For you it's a lot of work.

 

[ericmark]

While writing up posted so it would seem you have RCD and separate MCB and it would seem fault caused by a loose screw in the isolator. It is likely once swapped all will be OK. But the neon screwdriver is not an electrical tester. To test you need two connections using your body as a connection as the neon screwdriver does tells you there may be a fault, investigate further that is all. You must then use a proper tester.

 

[BillDawgy]

Thanks for your replies! I've tested the screwdriver on many appliances and the cooker isolation switch is the only one to show any electrical power whilst its breaker is off, its just weird and im wondering if it caused the fault in the first place.

No. As said - loose connection.

Would that really cause the wire to start melting about 2inches above the "loose connection"? Genuine question. To me that'd denote that the wire itself is getting too hot.

Thanks all, I will mention to the electrician that power still shows when the breakers off (im sure he'd notice it anyhows, be shocked otherwise pardon the pun)

Thanks to everyone for such fast replies

 

[bernardgreen]

Would that really cause the wire to start melting about 2inches above the "loose connection"? Genuine question. To me that'd denote that the wire itself is getting too hot.

The loose terminal generates the heat and the copper wire conducts the heat many inches along the cable inside the insulation.

 

[EFLImpudence]

It didn't start melting two inches from the terminal; it stopped melting there.

 

[333rocky333]

He will need to cut the burnt bit off even if its 2 inches or more , does it look like thats possible to do and still leave enough to connect, it is no good him just taping over the burnt copper.

A loose connection makes the copper get very hot, thus melting the insulation, over time it can creep up the cable easily 2 inches or more, bigger appliance loads like cookers and showers would heat up a loose terminal, a lot more than say a table lamp or a low load appliance

 

[Echo the husky]

I would also recommend replacing the switch with a quality brand such as Crabtree or MK. Those BG ones are rather cheap and nasty IMHO.

 

[BillDawgy]

Thanks guys for the knowledge electricians coming over to replace.

Once again thanks amillion!