Oven cable melted cable and casing - can't work out why

[SashaWu]

Good Morning,

Recently got a new oven - hooked it up the same as every other oven I've had. However the live wire seems to have melted, melted the plastic casing and blown the fuse.
https://i.Upload the image directly to the thread.com/bnOJTEW.jpg

Any thoughts?

 

[Wilko]

Hi - my thought is the L (red) has overheated, melted its insulation and likely the insulation on the green earth wire and created a short circuit which has blown the fuse. Or not, as it's hard to be accurate from just a pic, sorry. The question is why did it happen? A poor connection, an undersized cable for the load or a faulty oven are some reasons.

 

[SashaWu]

Thanks for your time and thoughts,

Sorry - I should have stated that the earth wire is unaffected, it's completely clean, I just moved it over a bit for the photo to get a better angle.

The thing is - this is the exact same setup as the previous oven, even down to the plastic cable box. The cable has been use for an oven for years.

 

[marconi]

Can you provide a picture of the rear side of the connector block?

 

[Pete999]

Loose connection?
What size in KWs is the cooker?
What size is the cable?
What size is the Fuse / Circuit breaker?

 

[Lucien Nunes]

The connection was insecure and of high resistance; the heat has melted the plastic housing and probably allowed the terminal to short-circuit to the metal casing. It's not clear whether your side of the connection was at fault or the internal one(s) hence Marconi's question. From the pattern of burning, I think it's your connection.

Common causes are crossed threads - the nut feels tight but isn't clamping the cable or cable not in position so that it creeps out from under the washer. Did you give it a good tug-test after fitting?

 

[Octopus]

Unless you tell us the rating of the oven and the cable size we are merely guessing

 

[Phil Thompson]

Id have gone with Lucien on my diagnosis.

 

[marconi]

I have enlarged the image you provided and can see what might be interpreted as a few strands of the red conductor which have been cut back to where the insulation ends. Thus there might have been a reduction in conductor cross sectional area. This would cause both an increase in current density and resistance - contributing to higher Ohmic heating (IsquaredR).

Is this the case?

The cable has a single CPC conductor and stranded live conductors so looks like 4 or 6 mm2 T/E.

Murdoch is spot on - we need to know the power of the oven, type of oven and size of cable.

 

[derek]

Was the appliance connected by a competent person as per the instructions on the oven?
It is easy for the unskilled to miss what would be obvious to a more experienced person and good termination of electrical equipment is a skill in itself.

 

[SashaWu]

Thanks all for your time and input,

Unfortunately I can't provide any more photos until later today when I'm home. I'll do this once I get in.

The model of oven is: Cannon by Hotpoint CH60ETC

It was installed by myself - I hooked it up per the little diagram on the rear of the device (which happened to be basically the same setup as my previous oven)

 

[Midwest]

Whatever, the OP is going to need a new termination block & possibly new pieces of oven internal wiring, and cutting back of the supply cable.

 

[Darkwood]

From the pic it looks like the plastic surround of the live terminal was broken when terminating, this may be why a secure tight connection was not made, you can see the clean fracture lines to the top left of the termination, this will not have come about from the overheating that led to the melting and shorting out.

Edit - change of mind, just seen where the missing part is, it is melted to the lid thus this fracture occurred after the OP open it post melting.. so as others have said, It comes down to a poor termination, as this was DIY were you trying to tighten the nuts with pliers or something similar or did you use the correct tool.

 

[SashaWu]

I used a hex screwdriver to tighten it, but thinking about it now - I may have tightened the black cable second, meaning the red may have been moved slightly.

Sorry for my lack of understanding - but if this was the case, why does a lose connection cause an issue like this. I was under the impression that as long as the metal is touching - it shouldn't matter how tight something is. The tightness was just to hold it in place.

 

[Lucien Nunes]

When two things touch, only a very tiny area of material makes contact - the rest of the surface is separated by a thin film of air / grease / tarnish. An electrical connection needs a minimum cross-section of copper-to-copper contact to carry the current, just like a cable needs a minimum cross section. When you clamp a terminal tightly, it squeezes out air and moisture and dirt as the metal cold-flows into position. Only once that has happened is the contact area large enough to carry the current involved here.

Also, as a terminal heats and cools (there's always some resistance, hence some heat) things move. Air molecules get at the surface of the copper and oxidise it, rendering points that were previously in contact insulative. Over time, the entire surface corrodes and the resistance slowly rises until this kind of runaway overheating occurs. When properly clamped, the metal-to-metal contact is gas-tight and no corrosion will occur at the interface.