Problem with 11KW Kiln

[Sally Mills]

Can you clever people advise as to why I might be having an issue with my ceramic kiln. I am a potter not an electrician so I will endeavour to give you what technical information I have.

11kw kiln running off a domestic board with its own 50A fuse on board in the house, 15m lenght of 10mm cable runs from house into my studio where it has its own board with a 62A breaker and 50A fuse.

The fuse in house is tripping when firing cycle reaches about 1060 degrees (kiln should reach 1250 degrees). It used to reach 1180 degrees before tripping but over a few years it has been tripping earlier and early.

The firing cycle of the kiln goes up slowly, automatically over 5 hrs automatically turning on and off until the end when its on full load. Kiln fired fine with no tripping day before yesterday (post element change) when taken fast at full load to temperature, but when I did a slower firing last night it tripped.

Kiln engineer took a reading of 45Amp and there is a drop in voltage from 240 to 232 when under load.

I have had kiln serviced and elements replaced this week and tripping still happening so pretty certain its not the kiln.

Questions: Can anyone see an issue with the electric maths? Is the cable thick enough? Can a domestic fuse be upped to next level? Can cables get tired and old?

Any advise as to what to try next would be greatly appreciated as I'm soooo frustrated with this ongoing issue.

Cheers lovelies!

 

[Sally Mills]

6.5cm diameter. Nothing written on cable but here is a picture

View attachment 40712

Followed cable further down and found 2541M engraved onto cable!

 

[Octopus]

Followed cable further down and found 2541M engraved onto cable!

Keep looking and you may well find the core sizes on their too.

look for 4mm, 6mm, 10mm ....

 

[DPG]

As Murdoch says, the upfront RCBO doesn't seem necessary. Tel's idea of changing it for a fuse may be wise. After the necessary testing obviously.

 

[Sally Mills]

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What does the certificate for the fuse board change state?

Looks like the small CU has a RCD too ........... which one is tripping?

Wonder why the installer fitted an upfront RCBO ?

I asked electrician to replace board as the kiln cable had almost caused a fire inside the old plastic board (melted the casing a little)!! Scary!. He decided to put the RCBO in.

It is the CU (on the left in picture) on house board that trips. The RCD on the right is for the lighting circuit of house.

 

[ruston]

What did he say in his opinion had caused the cable to overheat .

 

[Sally Mills]

Did the spark not fit the SWA cable?

Cable fitted 10 years ago by newly qualified friend!! Always wondered if he did the right thing.

Really appreciate all your advice. Without finding the cause I have no business. Just though many heads better than one as I am desperate and I was right..you're all great. I'll make you all a cyber cuppa
l

 

[Sally Mills]

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What did he say in his opinion had caused the cable to overheat .

He thought the cable wasn't screwed in properly on old board...but it was all melted so not sure how he can tell.

 

[davesparks]

I think the 50A breaker is a bit tight for an 11kw heating load. At 232v that works out to 47.5A.

Only if it’s 11kW at 232V, if it’s 11kW at 240V then it’ll be less at 232V

 

[ruston]

Ok , that happens too .
If the wall that the cable is on is south facing and gets lots of sun , and judging as it is business use and is on for long periods - that cable is going to have a good start at getting preheated.
It could be as has been said before the insulation resistance has been compromised. , or the cable has been damaged by the first overheat incident.
If I were you I would get a reputable electrician to inspect and test this circuit .
Maybe someone on here is nearby.

 

[Andy-1960]

Only if it’s 11kW at 232V, if it’s 11kW at 240V then it’ll be less at 232V

True, but what the betting it’s rated at 230v

 

[Octopus]

Getting back to facts, what does the Electrical Installation Certificate, provided to you by the chap who fitted the new fuse board, state about the cable sizes?

 

[Richard Burns]

You have a circuit that has two 30mA RCDs in series.
It also has two 50A circuit breakers in series.
The one nearest the kiln is not tripping and the one at the house is tripping.
So long as the RCDs / MCBs are all working this indicates that the fault is before the RCD/MCB at the kiln, i.e. there is damage to the supply cable.
A 6.5mm (approx.) diameter inner core would be a 10mm² cable.

An insulation resistance test of the supply cable may highlight this; because the cable has been overheated it may mean the insulation has been damaged at some point further along the cable run, when the cable is in use and warming up (not overheating) the insulation may then fail and cause the RCD to trip (or possibly the MCB part of the RCBO).

 

[Sally Mills]

You have a circuit that has two 30mA RCDs in series.
It also has two 50A circuit breakers in series.
The one nearest the kiln is not tripping and the one at the house is tripping.
So long as the RCDs / MCBs are all working this indicates that the fault is before the RCD/MCB at the kiln, i.e. there is damage to the supply cable.
A 6.5mm (approx.) diameter inner core would be a 10mm² cable.

An insulation resistance test of the supply cable may highlight this; because the cable has been overheated it may mean the insulation has been damaged at some point further along the cable run, when the cable is in use and warming up (not overheating) the insulation may then fail and cause the RCD to trip (or possibly the MCB part of the RCBO).

That makes sense thanks Richard. The cable from kiln RCD/MCB to the house is 15 meters long and cost a blooming fortune to have installed. I assume if the is a problem along this run the whole length has to be replaced?


Your help is much appreciated. x

 

[Sally Mills]

If the cable is damaged do you think the non regulation bends (as Vortigern suggested) in the cable might be an area of weakness/earthing?

 

[The Ghost]

I personally would have gone the next size up on the cable as the cost of the cable difference is not that significant. If the cable is bent at too acute angle then I think it is possible under load over a certain time period to allow enough leakage to cause the tripping it takes only a very small current to actuate those trips. @Murdoch I was hedging my bets as I am not sure if that is a high integrity CU and the RCBO was within or without the aegis of the RCD. If it is in that is poor installation technique as regards certainly discrimination. As someone pointed out above, serial RCDs' abound which is very messy. Melty CUs' are usually the result of poor termination techniques of the cable in the CU especially the supply cable.