Tripping Rcd

[soulman]

Hi, Can anyone shed any light on what is happening here. 9kw single phase oven in commercial premises. Intermittent fault when opening the 45a D.P switch for the oven this trips the rcd. This is a split rcd board, the rcd side contains 3 rings & the oven unprotected side contains 3 lighting circuits & a free standing electric cooker. Rcd test o.k. Ir test are all above 4 Mohms. ramp test 27ma.

Cheers

 

[steptoe]

is the switch disconnecting the neutral before the Line?

some switches MUST be connected polarity correct

 

[soulman]

cheers steptoe, yes polarity is correct, cooker switch was changed by previous electrician, all termination were very loose, so retightened at switch and tightened all connections at board.

 

[Steve3948]

So it only trips when switching off, never when switching on.

 

[SLIPSHOD & SLAPDASH]

I have had this & heard of it a few times. Its the neutral switching first, but never worked out why it trips the rcd. Has happened on a bank of fluorecent fittings & on a garden water feature. No other faults or leakages.

 

[soulman]

Yes that is correct only when switching off. never when switching on.

 

[Evans Electric]

HHmmmm! Could try re-connecting the 45A sw the other way around ...ie: L through the N side , N through the L side. if the N is opening first .

Not sure why it would trip an RCD though .

 

[steptoe]

yer, have had it a few times and I reckon it was N disconnecting first, slight leak to E perhaps, just enough to upset the balance of equilibrium in the world ,

new switch may be an option to try, oh, and get one without a neon,

 

[ADS]

I'm not getting how it matters which conductor it switches first...........nor what a neon has to do with it.

 

[steptoe]

ok, so you have a small leakage to earth, suddenly you have no neutral, the current will still try and get out, possible using the leakage to earth,

imagine a hosepipe in general use with a small hole , a little water gets out, but not so much as affects its use, now block the end up and see how much water shoots out that little hole.

 

[ProDave]

yer, have had it a few times and I reckon it was N disconnecting first, slight leak to E perhaps, just enough to upset the balance of equilibrium in the world ,

new switch may be an option to try, oh, and get one without a neon,

That's the first time anyone has given a plausible explanation for what might trip an RCD at switch off.For that to happen, the circuit must have an inherent fault (leakage) but not enough to trip the rcd in normal use. But open the neutral, and then there's a much greater leakage from L to E.

Certainly if I ever encounter that, I'll try swapping the switch poles over, or even bypass the N switch so only the L is switched.

 

[ADS]

ok, so you have a small leakage to earth, suddenly you have no neutral, the current will still try and get out, possible using the leakage to earth,

imagine a hosepipe in general use with a small hole , a little water gets out, but not so much as affects its use, now block the end up and see how much water shoots out that little hole.

I'll give you that what you suggest is feasible.......but in the OPs scenario, all IR bigger than 4 million ohms - so no leak to earth.

 

[steptoe]

he hasnt tested the appliance,

it will be the appliance leaking, dunno about you, but Id be nervous sticking 1000v through someones appliance

:|

 

[ADS]

he hasnt tested the appliance,

it will be the appliance leaking, dunno about you, but Id be nervous sticking 1000v through someones appliance

:|

But the appliance is on the load side of the DP switch, so any leak to earth within it would not trip the RCD once the switch was opened.

PS I wouldn't be sticking 1000 volts through anything, let alone someones appliance B-)

 

[Riggy]

But the appliance is on the load side of the DP switch, so any leak to earth within it would not trip the RCD once the switch was opened.

True once both poles of the switch are open.

I like this theory, I'd like it even more if the Zs was fairly high so that in normal use it's much easier for the current to return via the neutral rather than the fault.

 

[professional]

I would suggest testing the oven with a leakage meter (around phase and neutral) under normal working conditions.

The issue of RCD`s tripping when switching circuits/appliances on/off has been around for years. I`ve had quite a few Memera 21 boards that would trip instantly if the RCD was reset whilst the mcb`s were still on the on position, yet the same rcd reset and worked fine if the mcb`s were switched on one at a time.

 

[kme]

As have I mate.

ADS - its true that leakage of the appliance doesn`t matter once the switch is open on both poles!

But if N has opened before L, the scenario Steps described so eloquently will occur.

 

[ADS]

As have I mate.

ADS - its true that leakage of the appliance doesn`t matter once the switch is open on both poles!

But if N has opened before L, the scenario Steps described so eloquently will occur.

I concede.

 

[soulman]

Hi, well i swapped over connections within the 45a d.p switch, still tripping rcd, the oven would run for days and be fine when turned off at the switch on the oven itself, only when switching off at d.p switch. So i have replaced the d.p switch which wasn't really suitable, for a 63a rotary isolator, surface mounted the cable in trunking as it was fed through metal studwork. and put the circuit onto the non rcd protected side.

Cheers

 

[steptoe]

Hi, well i swapped over connections within the 45a d.p switch, still tripping rcd, the oven would run for days and be fine when turned off at the switch on the oven itself, only when switching off at d.p switch. So i have replaced the d.p switch which wasn't really suitable, for a 63a rotary isolator, surface mounted the cable in trunking as it was fed through metal studwork. and put the circuit onto the non rcd protected side.

Cheers

that'll deffo stop it tripping out the RCD,

but why ?

dont you think the RCD was tripping for a reason?