RCD tripping dilemma

[2BobSparky]

Hi all,

Cam across a tripping 63A RCD, which is protecting pretty much everything in an office and even the lighting!

The first guy who came out seemed to think that perhaps a spur feeding a AC unit was the culprit and proceeded to change it over, but I didn't really see the point in that.

I reckon it's something to do with the light as I know from the past that lighting is notorious for causing nuisance tripping when being protected by an RCD.

Basically, I would like to be able to isolate the cause of the main RCD tripping, but unfortunately no MCB popped before the RCD!

 

[Plonker 3]

Welcome to the world of fault finding. A MCB wont necessarily trip because the RCD is

Could be one of thousands of things causing this. Is there a particular pattern to when it is tripping like when they boil the kettle, at night when no one is in the building.

Have you ramp tested the RCD to prove that is fine?

What have you done already to try and find the fault?

 

[telectrix]

fluorescent lighting is notorious for leakage. too many fittings on a RCD will cause it to trip.

 

[2BobSparky]

A MCB wont necessarily trip because the RCD is

Is there a particular pattern to when it is tripping like when they boil the kettle, at night when no one is in the building.

Have you ramp tested the RCD to prove that is fine?

What have you done already to try and find the fault?

It would be nice if the MCB tripped first though and considering RCDs are constantly referred to as "secondary protective devices" I thought this might be a sign as to what could be causing the RCD to go, but alas it's not that easy of course.

Doesn't seem to be any clear pattern as to what is happening, conflicting reports from different people in the office. Initially though, it appears as though they have gone to switch on all the AC units in the office and going to turn on the last one that was left off has caused the RCD trippage.

Never heard of "ramp testing", but I am familiar with a RCD test done as apart of an inspection and test, is this what you mean?

 

[2BobSparky]

fluorescent lighting is notorious for leakage. too many fittings on a RCD will cause it to trip.

When you say "leakage" I think I am getting the function of an MCB and a RCD mixed up.

My understanding of an RCD is by the measurement of current on both Active and Neutral wires and any imbalance will cause the RCD to activate. With this in mind, how does earth leakage come into the equation, which I thought was something more associated with an earth leakage MCB.

 

[Plonker 3]

It would be nice if the MCB tripped first though and considering RCDs are constantly referred to as "secondary protective devices" I thought this might be a sign as to what could be causing the RCD to go, but alas it's not that easy of course.

Doesn't seem to be any clear pattern as to what is happening, conflicting reports from different people in the office. Initially though, it appears as though they have gone to switch on all the AC units in the office and going to turn on the last one that was left off has caused the RCD trippage.

Never heard of "ramp testing", but I am familiar with a RCD test done as apart of an inspection and test, is this what you mean?

A ramp test will check how much leakage is needed before the RCD trips. So ensuring the RCD isn't tripping at too low a leakage.

I think maybe you are out of your depth with this, and walk away now or get someone with more experience to help you.

 

[Mr.Ian]

I'm sure AC units (air con???) like fridges/freezers can trip RCDs, there aren't a few of these units and rfc's and lighting all on one RCD is there?

 

[truckster]

If you have got flourescent lighting, a/c units and computers all on one RCD you have probably a fair amount of leakage in normal use, have you got a milli-amp clamp meter? Probably need to divide the circuits across two or more rcds or swap the MCBs out for rcbos and lose the front end RCD

 

[Deleted member 9648]

FFS...... do an IR test.....:banghead:

 

[2BobSparky]

So nobody would suggest taking the MCB feeding the AC units off of RCD protection to prove if in fact, like the office workers seem to believe, that AC units are the cause of the problem?

If the MCB trips without RCD protection then clearly that's the problem isn't it?

 

[Octopus]

So nobody would suggest taking the MCB feeding the AC units off of RCD protection to prove if in fact, like the office workers seem to believe, that AC units are the cause of the problem?

If the MCB trips without RCD protection then clearly that's the problem isn't it?

To save messing around with the CU why don't you simply recommend the air con is left off for a specific period!! or could you use a clamp meter on the Air con circuit? do a ramp test on the RCD, IR the circuit?

Problems like this are frustrating and yes the employees may have a point but you need a clear approach to fault finding or I suspect you'lll go round and round in circles

 

[tony mc]

I would be clamping the tails to see what sort of leakage you have.

By what has been said so far it is possible that the RCD is in fact only doing its job and detecting the leakage.

 

[2BobSparky]

To save messing around with the CU why don't you simply recommend the air con is left off for a specific period!! or could you use a clamp meter on the Air con circuit? do a ramp test on the RCD, IR the circuit?

This is not feasible with the client involved and also the temperature in their office! This is also why something like an IR test is not possible at this very moment as we ware talking about a dead test, which means switching their CU off, it's simply not going to happen during business hours in the West End of London.

So anyway, in the end the RCD MCB was taken off of RCD protection and has been left alone to see if it will inf act trip itself.

 

[Strima]

Looks like it's going to be out of hours testing then. The RCD tripping shows there's a fault somewhere which needs identifying and rectifying, I personally would not have left the circuit without RCD protection as the designer deemed it necessary to have it.

Even a quick test taking 15 minutes could highlight problems and reduce the amount of downtime the office has. Protection is there for a reason.

 

[Darkwood]

When you say "leakage" I think I am getting the function of an MCB and a RCD mixed up.

My understanding of an RCD is by the measurement of current on both Active and Neutral wires and any imbalance will cause the RCD to activate. With this in mind, how does earth leakage come into the equation, which I thought was something more associated with an earth leakage MCB.

Do a little revision on how rcd's work first before you address the problem, its clear from your confusion here your going into a problem slighty blind. For a heads up your are correct in that it monitors any different between the feed current to the load and the return current from the load, if it differs the current has leaked elsewhere and usually to earth.

Also check are the light electronic ballast, how many computers are in the offices etc, read up on natural background leakage currents and their causes.

 

[Des 56]

That well known saying is often used in the electrical industry,

"Its better to be sorry than safe"
or is it
"The Show must go on"




Regardless of safety

 

[2BobSparky]

I'm sure AC units (air con???) like fridges/freezers can trip RCDs, there aren't a few of these units and rfc's and lighting all on one RCD is there?

Yep, that's how it is.

 

[Darkwood]

Looks like it's going to be out of hours testing then. The RCD tripping shows there's a fault somewhere which needs identifying and rectifying, I personally would not have left the circuit without RCD protection as the designer deemed it necessary to have it.

Even a quick test taking 15 minutes could highlight problems and reduce the amount of downtime the office has. Protection is there for a reason.

A tripping RCD does not always mean there is a fault present, alot of modern office equipement have electronics that have a designed leakage current to earth hence the term High Earth Currents which must be accounted for in design when you have a no' of computers or light fittings with electronic ballasts, in the OP case i would first do a earth current test to establish if there is already a high leakage which may be related to designed leakage as opposed to fault leakage.

 

[Strima]

Very true mate, trying to read a post and reply with two screaming kids and a nagging wife running around doesn't help...

 

[vernam616]

Why not call an electrician?