Another question for the more wise??

[DaveS79]

Evening

Just home from job. Was there a week ago to install a replacement CU. At time completed full range of tests. All IR readings fine. Customer contacted me to say every time uses Oven RCD trips. No problem sounds familiar.....dodgey old oven needs replacing. As like to be 100% before advising replacement passed by this evening to do few tests to confirm. When got there she said on using the hob once, which is also on same circuit as oven rcd tripped also. So thinking could possibly be circuit problem disconnected from CU and appliances and circuit fine. I IR tested the cable to oven with all switches fully on, no problem high readings I tested the cable to hob and individually turned on each ring and two of the rings giving me IR readings between N-E of 0.05M. Thinking this must be the problem as these readings should be low enough to trip RCD? I disconnected hob and powered oven up, 2 minutes later RCD still tripped...this stage bit baffled. Did a ramp test and tripped at 21ma.....this seems quite low to me so tried on the other RCD which had a 25ma ramp test. Still tripped. I finally left hob connected and disconnected oven, whacked on full power and was holding for the 5 minutes I was there. I have advised her before purchase of oven just use hob for a week and see how she gets on. Just a bit baffled by the readings I got and also that she reported the hob tripping RCD on one occasion. Any heads up be appreciated

 

[Murdoch]

Ovens and often specifically the element are key items along with kettles for tripping RCDs 

do you have an earth leakage clamp meter?

 

[DaveS79]

Ovens and often specifically the element are key items along with kettles for tripping RCDs 

do you have an earth leakage clamp meter?

Hi Murdoch

I do yes, but tbh very rarely have had need for it so hasn't been on van. Will make sure have it in future. Is a IR reading at terminals of cookers not a great way to test then? Also am still abit miffed by the o.o5M readings was getting on the hob. My understanding is that readings this low should trip RCD straight away?

Cheers

 

[SPECIAL LOCATION]

Hi Murdoch

I do yes, but tbh very rarely have had need for it so hasn't been on van. Will make sure have it in future. Is a IR reading at terminals of cookers not a great way to test then? Also am still abit miffed by the o.o5M readings was getting on the hob. My understanding is that readings this low should trip RCD straight away?

Cheers

Think you maths is a bit lacking IMHO..

i.e.

1.0Meg = 1,000,000ohms

and 0.05Meg ohms  =   50,000ohms    or  50kohms

30ma @ 230v is around 7,666.66ohms  or 7.7kohms

30ma @ 240v is around 8,000ohms  or 8kohms

an IR reading of 0.00Meg can still be above a sufficiently low resistance to trip a 30ma RCD.  

As 0.0099 Meg is 9,999ohms  which reads as 0.00Meg on your IR Test..

e.g. even a 10kohm resistance to Earth may not trip a 30ma RCD..

as  230v / 10,000ohms  = 23ma!!  

Is this just a dual RCD board or full RCBO?

:C

 

[DaveS79]

Think you maths is a bit lacking IMHO..

i.e.

1.0Meg = 1,000,000ohms

and 0.05Meg ohms  =   50,000ohms    or  50kohms

30ma @ 230v is around 7,666.66ohms  or 7.7kohms

30ma @ 240v is around 8,000ohms  or 8kohms

an IR reading of 0.00Meg can still be above a sufficiently low resistance to trip a 30ma RCD.  

As 0.0099 Meg is 9,999ohms  which reads as 0.00Meg on your IR Test..

e.g. even a 10kohm resistance to Earth may not trip a 30ma RCD..

as  230v / 10,000ohms  = 23ma!!  

Is this just a dual RCD board or full RCBO?

:C 

Thanks. Dual

 

[SPECIAL LOCATION]

Dual

Well there's part of you problem..

how many circuits per RCD..?

how can you discount other residual current causing problems..?

i.e.  3 circuits each with 10ma of residual leakage all fine and ok on an individual RCD / RCBO.

but stick them on a shared RCD..  and you open a can of worms for all shorts of potential problems..

A lot of oven manufactures used to recommend not putting them on RCD's due to known leakage issues.

:C

 

[Murdoch]

Hi Murdoch

I do yes, but tbh very rarely have had need for it so hasn't been on van. Will make sure have it in future. Is a IR reading at terminals of cookers not a great way to test then? Also am still abit miffed by the o.o5M readings was getting on the hob. My understanding is that readings this low should trip RCD straight away?

Cheers

With nuisance tripping, ESPECIALLY on up front RCD boards or dual RCD boards its always a good idea to get a reading of the earth leakage as part of your initial tests to get a handle on whats going on. 

Then in the case of the oven, watch the reading as the oven gets hot ............

 

[DaveS79]

Well there's part of you problem..

how many circuits per RCD..?

how can you discount other residual current causing problems..?

i.e.  3 circuits each with 10ma of residual leakage all fine and ok on an individual RCD / RCBO.

but stick them on a shared RCD..  and you open a can of worms for all shorts of potential problems..

A lot of oven manufactures used to recommend not putting them on RCD's due to known leakage issues.

:C

It's got only 2 circuits on that side of RCD. One being the shared oven/hob, the other being a ring circuit which I totally disconnected. I also switched it to the other RCD and same thing happened.

As Murdoch said I should have put the leakage detector on it

With nuisance tripping, ESPECIALLY on up front RCD boards or dual RCD boards its always a good idea to get a reading of the earth leakage as part of your initial tests to get a handle on whats going on. 

Then in the case of the oven, watch the reading as the oven gets hot ............

 

[Murdoch]

It's got only 2 circuits on that side of RCD. One being the shared oven/hob, the other being a ring circuit which I totally disconnected. I also switched it to the other RCD and same thing happened.

And that is precisely why I think that RCBO boards should be mandated ..........

 

[Geoff1946]

I would put money on it being mineral insulated elements in the oven. 

If they aren't used for a while the mineral insulation absorbs moisture from the atmosphere.

Whilst the element is cold the moisture is dispersed along some length of it, and still gives a good insulation resistance when checked.

When the element is powered up the increasing temperature drives the moisture towards the cooler ends, where the greater concentration results in a much lower resistance, and consequent rcd trip.

If you can keep the element hot for long enough, (perhaps on a non-rcd circuit) you may drive out enough moisture to cure the problem, but the "proper" answer is a new element.