Oven tripping RCD occassionally when on for long time

[Casterbridge]

Oven about 2 years old fromn new - previously running fine.

Tenant now states that sometimes, particularly when oven is on for long time that the RCD trips. They have oisolated it to the oven and not the hob (on same circuit, 32A MCB), and say that the hob does not need to be on for it to trip. I have taken a look at all the wiring from the CCU to the appliance and everything seems fine, all screws are tight and no evidence of cable overheating.

If it's the appliance, what possible things could suddenly cause this intermittance fault?

 

[PrenticeBoyofDerry]

Likely to be a heating element.

 

[Casterbridge]

Likely to be a heating element.

Thank you - can you talk me through how that could happen - i.e. how it trips the RCD at high temperature?

 

[PrenticeBoyofDerry]

Heating elements are vulnerable to earth leakage, this is what the RCD is detecting, is not unusual for any element set at a lower temperature not to produce enough earth leakage to trip the RCD.
Your RCD is likely to be trip rated at 30mA, but they tend to actual trip at around 26mA. Once this value of earth leakage is reached then the RCD will trip, but the value is less than that it will stay set.
It tends to be fan element that goes first, but other must be considered.

 

[Casterbridge]

Heating elements are vulnerable to earth leakage, this is what the RCD is detecting, is not unusual for any element set at a lower temperature not to produce enough earth leakage to trip the RCD.
Your RCD is likely to be trip rated at 30mA, but they tend to actual trip at around 26mA. Once this value of earth leakage is reached then the RCD will trip, but the value is less than that it will stay set.
It tends to be fan element that goes first, but other must be considered.

1. That's really helpful thanks. As it doesn't happen all the time and only when the oven has been on for a long time - would it be difficult to diagnose? Is it likely that insulation resistance would not indicate issue with particular part once fauly occurs, or could this be detected when cold?

2. Given the description of the fault, is it unlikely to be thermostat or fan itself?

 

[bernardgreen]

It might be steam / water vapour from the food getting into the back of the oven and condensing on electrical wiring.

 

[Casterbridge]

It might be steam / water vapour from the food getting into the back of the oven and condensing on electrical wiring.

OK, the plot thickens.

Would a 250V insulation test of the removed element likely confirm/eliminate it as the cause of the fault described?

 

[flyingsparks]

It might be steam / water vapour from the food getting into the back of the oven and condensing on electrical wiring.

OK, the plot thickens.

Would a 250V insulation test of the removed element likely confirm/eliminate it as the cause of the fault described?

Yes an element which is obviously 'on its way out' would have a low IR reading. Probably lower than 0.5M even when cold.

 

[PrenticeBoyofDerry]

An insulation resistance test on the element would help diagnose the fault, and a continuity test should confirm the elements power output.
There are many situations, components and faults on an item of electrical equipment and the hard wiring of an electrical installation that can cause the RCD to trip, it is often a case of elimination this can be frustrating but through experience when a appliance with a heating element is causing an RCD to trip, I routinely test the element, it could be that there are a number of elements on the appliance, I tend to see the fan element go more often than others.
But don't rule out a faulty with the RCD, but this would require a ramp test to test that it is working to somewhere near it's rating.

 

[bernardgreen]

Finding the reason for random RCD tripping is a black art.

There is always the possibility that the fault is not in the oven but is a resistive neutral to earth fault elsewhere which is diverting neutral current from the oven away from the RCD sensor.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

With a resistive neutral to earth fault ( as apposed to a dead short ) a large current ( several amps ) will be needed to flow into the neutral bar before there is enough diverted to trip the RCD. If the oven is the largest load then it is the most likely apparent cause of the fault.

It may be an item that is only plugged int while the oven is in use.

 

[Casterbridge]

Thanks for everyone's kind help and advice!!

The bottom double element showed signed of charring.

Untitled

• Casterbridge
• 16 Jan 2014
• 1

Image is upside down (you are actually looking at the bottom element)

The second (orange) spade connector is quite loose - it connects, but not a tight connection.

Resistance for both elements were around 28 ohms

Insulation Resitance between all four live connectors and earth varied between 28Mohms and 38Mohms

My thoughts are that when the oven is on for a time, the loose connector becomes problematic and shorts to earth, hence the charring.

How did the connector become loose? There is cooking grease that has made its way past the insulation packing from the inside of the ovcen to the back of the oven housing and sits on the insulation when the back is off. When I touched the insulation the grease was still damp on the insulation. I suspect that this could have dripped on the bottom connector and started the problem???

Any thoughts??