Zanussi double over tripping RCD

[Tony40065]

Hi, my son has a double oven that trips the main RCD but only when both ovens are on AND have heated up.
Initially we thought it was the top oven element because he noticed it when he used that oven, so we replaced it but but it turns out it was just that the main oven always happened to be on when he used the top oven, it so it still trips out.
I wondered if it was overloading the circuit but it’s on a dedicated 32a mcb so should be fine, plus the breaker is never tripped, only the main RCD.
I’m loathed to shell out for another two elements because I can’t see why an earth fault would only show if both ovens are hot, but I’m also not an oven expert.
Is this a typical fault that is likely to be one of the other elements or are we looking at something more sinister?
Thanks

 

[Wayners]

Check cable running from fuse box to oven is correct size..
Check cable is in good condition and any joints are in good condition.

I remember an oven having lighting cable to a poor condition junction box. Electrician run a bigger new cable from fusebox direct to oven. Customer kept burning the roasts and cakes as it run so well and never tripped

 

[Rodders53]

Earth leakages from L and/or N to earth will be cumulative...

Mineral insulated heating elements may as they age start to leak to earth, seals fail they become hygroscopic and the elements move in the insulation with expansion / contraction. Other devices may/will have deliberate leaks to earth via filters to prevent Radio-Frequency Interference (inc the oven motor and thermostat switches possibly).

One RCD supplying multiple devices may be close to tripping when another device (oven element) gets switched on and.... trip (aka nuisance tripping).

Relatively expensive test kit can measure this and help identify the cause(s) and check the RCD is working as designed. It may be the RCD has become over-sensitive and trips too early.
In dual RCD situations it may be possible to move a circuit from one to the other to share the leakage load more equally (or not).

Without access to test kit (insulation tester at least) it will be difficult to eliminate the oven and it's wiring.

It's one reason why modern CU best practice is to have individual RCBOs for each circuit.

Cue Eric for chapter and verse.

 

[Harry Bloomfield]

Is this a typical fault that is likely to be one of the other elements or are we looking at something more sinister?

You need to begin, by understanding, how an RCD works....

They compare the current travelling through the L, with that through the N. If there is a difference of >30mA, then it should disconnect the supply/trip, because there is a leakage of current somewhere. The leakage can be either L to E, or N to E, and can be from more than one source, and accumulation of leakages, as from your two oven elements, or more than one appliance on the circuit. Some appliances are fitted with mains filters, where you can get some leakage.

As you seem to have an RCD, protecting more than one MCB, more than one circuit, it might be worth your while completely isolating all the circuits on that RCD, apart from the oven, then seeing if the RCD still trips, when both elements are in use.

The only effective way to check an element for leakage, is using a specialised, 500v insulation tester.

 

[Tony40065]

It hadn’t occurred to me that the earth leakage would be cumulative. There isn’t a lot being fed from the RCD, only the oven, a ring main, and the central heating. The supply to the oven is 6mm from a dedicated mcb, but the consumer unit and components are 20+ years old. I checked the manufacture date of that specific unit and it was 2002 - 2009, but it’s wired in black and red so I’m assuming closer to the earlier date.
I’ll turn everything else off and try it on its own to see if that makes a difference then work from there.
Thanks for your help.

 

[Harry Bloomfield]

It hadn’t occurred to me that the earth leakage would be cumulative. There isn’t a lot being fed from the RCD, only the oven, a ring main, and the central heating. The supply to the oven is 6mm from a dedicated mcb, but the consumer unit and components are 20+ years old. I checked the manufacture date of that specific unit and it was 2002 - 2009, but it’s wired in black and red so I’m assuming closer to the earlier date.
I’ll turn everything else off and try it on its own to see if that makes a difference then work from there.
Thanks for your help.

OK, just keep in mind that the MCB's, will only isolate the L, the N in each case remains connected and leakage N to E can also trip RCD's.

 

[jj4091]

What is the model number

 

[Tony40065]

ZOD35517XA

 

[bernardgreen]

It is possible that a Neutral to Earth fault on another circuit ( not the oven circuit ) is creating a mis-match at the RCD when the oven(s) are drawing current.

These can difficult to locate as it requires testing for Neutral to Earth in every circuit on the RCD

.

The oven(s) being the load

 

[Tony40065]

Thanks. I’ll isolate the L&N on all circuits apart from the oven and see if it’s still trips. If not I’ll reconnect each one separately to see if the fault returns so I can identify which circuit is causing the problem.

 

[jj4091]

A bit of a long shot but also check the steam hose isn't leaking and blowing steam onto an electrical component or has condensation dripping off it on to something