Discuss Rcd's that fail to operate in less than 40 m/s in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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Hi All

Ok...after many years of testing rcd trip times...having some failed or faulty rcd's....we are starting to come across perfectly good rcds / rcbo's . .from various makes....that wont trip at all..or wont trip in under 40 m/s.

about a year ago...in a domestic property..we fitted a new dual rcd c/u . ..1 rcd refused to trip either with test button or tester...we swaped the 2 over...now the other one wouldnt trip....
in the end we found a pressure fault between n - e at a spur we didnt know existed (it ran an old back boiler pump)....we informed the niceic of this via the help line and were advised we should contact the maker of the rcd as it seemed a n - e fault was holding the rcd on rather than making it operate...odd....

now this week in an industrial / commecial unit..we have discovered that rcbo's covering 13A socket outlets around the building..in both offices and the shop floor are failing to operate either at all or well over 40m/s even at x5.

in the same week...we found an exisitng rcd in another domestic that failed to operate if the boiler spur (which was covered by this rcd in a dual rcd board) was switched on....external boiler..less than 5 years old....brand new controler....
switch the spur off...and all is good.... (this house does have a pv circuit...but it was switched off and both ac and dc isolators were also off)

Now i understand that DC being leaked onto a circuit can cause and rcd to not operate...

ive checked with some wholesalers...and type A or type B rcd's are seemingly hardly ever fitted....mk are making theirs obselete...though hager are listing a type A ... well over £100 ...

My question is really ...HOW IS EVERONE GETTING THESE RCD'S / RCBO'S TO COMPLY WITH THE 40M/S STANDARD..??

no one is fitting the type A or type B ones ....and more and more inverters are being fitted to stuff these days...even in some washing machines now...not just pv and motor speed controls....im assuming this is the cause...

but the question is what do we do about it...how do we get these to comply....or do we not bother..and just note down the failure in our observations...and deviations from 7671...?

is this the begining of the end for rcd protection..?....

in some tt systems i work on...where the rcd is almost total protection...what happens when it wont operate..?

all thoughts greatfully accepted

cheers

vitoboy
 
eh.... ?... so a n - e fault on an unrelated circuit can hold a rcd or rcbo on on another circuit...


why...?....


on a dual rcd board....a n-e fault on one side can cause the rcd on the otherside not to operate..?....

ive seen interaction between 2 rcd's in a dual rcd board before...ie both trip at the same time..or one after the other.....with this it was an almost undetectable n-e fault my megger didnt even pick it up....but we checked each circuit..then each bit of wire.....and in the end found a broken jb...with 2 termainals not touching..but so close...maybe causing capacitance....

but what do you do in a commercial install with over 100 circuits.....when you do an install...tell them you must test the whole place to fit 4 new sockets..?...

cheers

vito
 
Hi All

Ok...after many years of testing rcd trip times...having some failed or faulty rcd's....we are starting to come across perfectly good rcds / rcbo's . .from various makes....that wont trip at all..or wont trip in under 40 m/s.

about a year ago...in a domestic property..we fitted a new dual rcd c/u . ..1 rcd refused to trip either with test button or tester...we swaped the 2 over...now the other one wouldnt trip....
in the end we found a pressure fault between n - e at a spur we didnt know existed (it ran an old back boiler pump)....we informed the niceic of this via the help line and were advised we should contact the maker of the rcd as it seemed a n - e fault was holding the rcd on rather than making it operate...odd....

now this week in an industrial / commecial unit..we have discovered that rcbo's covering 13A socket outlets around the building..in both offices and the shop floor are failing to operate either at all or well over 40m/s even at x5.

in the same week...we found an exisitng rcd in another domestic that failed to operate if the boiler spur (which was covered by this rcd in a dual rcd board) was switched on....external boiler..less than 5 years old....brand new controler....
switch the spur off...and all is good.... (this house does have a pv circuit...but it was switched off and both ac and dc isolators were also off)

Now i understand that DC being leaked onto a circuit can cause and rcd to not operate...

ive checked with some wholesalers...and type A or type B rcd's are seemingly hardly ever fitted....mk are making theirs obselete...though hager are listing a type A ... well over £100 ...

My question is really ...HOW IS EVERONE GETTING THESE RCD'S / RCBO'S TO COMPLY WITH THE 40M/S STANDARD..??

no one is fitting the type A or type B ones ....and more and more inverters are being fitted to stuff these days...even in some washing machines now...not just pv and motor speed controls....im assuming this is the cause...

but the question is what do we do about it...how do we get these to comply....or do we not bother..and just note down the failure in our observations...and deviations from 7671...?

is this the begining of the end for rcd protection..?....

in some tt systems i work on...where the rcd is almost total protection...what happens when it wont operate..?

all thoughts greatfully accepted

cheers

vitoboy


Vito I'm confused, what's a pressure fault?
 
An RCD is tested for compliance without any load or circuit connected, the "faults" you describe are circuit faults or equipment designs that inhibit RCD operation and are not faults on the RCD.

If you have a neutral to earth fault on a circuit then a proportion of the test current applied to the line earth loop will divert via the neutral and this will cause inaccurate readings as the full applied test current will not be seen at the RCD.
However this is again a circuit fault not an RCD fault.
 
Please explain as your statement is confusing

As how can a RCD be prevented from tripping if a circuit is not protected by an RCD

If there is a neutral earth fault on the non RCD side the test current could partially flow through the fault and return on the neutral conductor.

edit: didn't see Richards post when I posted.
 
Last edited:
Hi Vito,

High Ra on a TT can prevent RCDs operating as they should. Had one recently where it tripped x5 in something like 80mS. The earth was PME'd and all RCD tests are now spot on.

You say that numerous installs have recently had this problem. Could be a fault with your tester perhaps. Hope it isn't like, but worth thinking about.
 
Vito I'm confused, what's a pressure fault?

Think he is referring to cables being crammed in to tightly in a back box, and when the face plate is screwed back it causes the conductors, albeit insulated, to press hard against each other. I have known poorly dressed cables to trip an RCD, even when the insulation all looks fine.
 
ok...

i know that we are testing the circuit...and the rcd....without load....

But....do we not take into account that as soon as the load is applied...those tests mean nothing....

we fit an rcd / rcbo to a circuit...we test it in isolation of what its circuit is feeding...and say its all good...
but the moment the load is applied...the rcd will not operate....

if its a dual rcd board...then the whole of the circuits on that rcd side are now not protected by the rcd....

is this right...???

why do we fit these rcd's & rcbo's .....if once the load is applied..they do not work.....even if a circuit thats not on the rcd side of a c/u has a fault...we dont know this because we have only tested the circuit we are working on.......if we test our circuit with all others isolated...is this a true test.....


i think im right in saying this..unless im missing something obvious....which is not unknown...


cheers for the replys so far


vitoboy
 
ok...

i know that we are testing the circuit...and the rcd....without load....

But....do we not take into account that as soon as the load is applied...those tests mean nothing....

we fit an rcd / rcbo to a circuit...we test it in isolation of what its circuit is feeding...and say its all good...
but the moment the load is applied...the rcd will not operate....

if its a dual rcd board...then the whole of the circuits on that rcd side are now not protected by the rcd....

is this right...???

why do we fit these rcd's & rcbo's .....if once the load is applied..they do not work.....even if a circuit thats not on the rcd side of a c/u has a fault...we dont know this because we have only tested the circuit we are working on.......if we test our circuit with all others isolated...is this a true test.....


i think im right in saying this..unless im missing something obvious....which is not unknown...


cheers for the replys so far


vitoboy

Hi dude,the two highlighted parts of your post,are two separate parts of test and inspect,with reference to domestic properties.

The specific tests,as laid out,in the new testament:1eye: deal with those separately.

It is accepted,that an RCD can pass the requisite tests,as can the circuit it protects,but differing results can then be achieved,when working together.

It is also accepted that as independent current using items,on a circuit,such as a ring final,are functioning,the RCD will react differently to any further occurring conditions.

In short...you may be worrying too much,fella.

I will swap your rambunctious RCD's,for the £60,000 horsebox living area i have had to strip out,due to someone fitting an 8.5kw shower in a vehicle which has a 16A hook-up,and telling the customer to "keep trying it till it works..." :dead:
 
Hi dude,the two highlighted parts of your post,are two separate parts of test and inspect,with reference to domestic properties.

The specific tests,as laid out,in the new testament:1eye: deal with those separately.

It is accepted,that an RCD can pass the requisite tests,as can the circuit it protects,but differing results can then be achieved,when working together.

It is also accepted that as independent current using items,on a circuit,such as a ring final,are functioning,the RCD will react differently to any further occurring conditions.

In short...you may be worrying too much,fella.

I will swap your rambunctious RCD's,for the £60,000 horsebox living area i have had to strip out,due to someone fitting an 8.5kw shower in a vehicle which has a 16A hook-up,and telling the customer to "keep trying it till it works..." :dead:



mmm...maybe i is....maybe im not......

all im saying is....

are we fitting these things because of their ability to get dissconnection times....

if so....are they working...or...not....

id love the horsebox.....if the 16A mcb stays in...all good.....have a short shower...im sure they put a type d in anyway...put in an rcd and a tt stake..and all good...........................i guess..

got to go...

ive an rcd factory to buy....

cheers

vito
 
about a year ago...in a domestic property..we fitted a new dual rcd c/u . ..1 rcd refused to trip either with test button or tester...we swaped the 2 over...now the other one wouldnt trip....
in the end we found a pressure fault between n - e at a spur we didnt know existed (it ran an old back boiler pump)....we informed the niceic of this via the help line and were advised we should contact the maker of the rcd as it seemed a n - e fault was holding the rcd on rather than making it operate...odd....

I might be missing something here. Was the spur off the RCD protected circuit? If that were the case, the RCD should trip when energised. If not, it shouldn't affect that circuit, and the RCD should work normally. Either way, that would suggest the RCD is the problem.
 
Hi guys first time poster allways seem to find the answers I'm looking for on her but not this time so I've joined.
Today I went to a job where the resident was claiming nuisance tripping maybe once ever coupl weeks on the sockets ring which is protected by a wylex 32amp 30ma rcbo I've carried out an rcd trip time test and found at 1/2 180 the rcbo is actually tripping at 39ms this is something I haven't seen before is my assumption that the rcbo is faulty as it shouldn't be tripping at all on a 1/2 test and it's as simple as that.
Max Zs reading 0.49
Any words or wisdom or it there is something I'm missing to be point in the right direction would be much appreciared
 
If your trip tests were done with no external loads on the circuit then they do reveal the RCD is faulty and replacing it should resolve the symptoms.
However there should not normally be leakage up to 15mA on a circuit in a house so it may be worth checking that your IR results for the protected circuit are not significantly low, it is probably a minor peak in current since it is intermittent but worth checking the circuit is not damaged.
 
Hi Richard,
All other test results were as expected Until I got the the rcd times it didn't trip at 0 degrees but did at 180 repeated the test 3 times at 1/2 as I didn't quite believe it any reason the rcbo wld be overactive just one way? This is the only thing that really bothered me in just putting it solely down to a faulty rcbo, although nothing was operating I didn't disconnect all loads I must admit
 

Reply to Rcd's that fail to operate in less than 40 m/s in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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