RCBO timing too high

shocked · Mar 4, 2014

Hi All
just done a c/u change fitted with 8 RCBO's and when testing the trip times 2 of the RCBO times were too high (40a 30ma =40.8 ms . 32a 30ma = >40 ms )on X5 ? changed the 2 x trips and retest exactly the same times ?! disconnect the circuits and retest trip times perfect, ok so retest circuits all ok ??! phone control gear and speak to tech it could be the cooker hood (double insulated ) or the w/machine, they don't like them. in my case it was the (40a) electronic shower (32a) combi boiler .
just letting you know
John

mechelec · Mar 9, 2014

MDJ said:
Oh Dear, why do you think these trip times are in place? regardless of whether you think 54ms isn't a problem or not the allowed limit is 40ms and trying to convince someone here 54ms is acceptable is actually wrong and to be honest could be considered illegal. Imagine if someone got killed from a trip which went out at 54ms on times 5 and the spouse of that person said they read on here it was fine. forget all that nonsence, disconnect cabling from the trip and ramp it, end of, sigh.

Which law would he be breaking then out of interest ?

Richard Burns · Mar 9, 2014

At 40ms trip time the voltage (and current) will have peaked four times, at 54ms there are another two chances to kill someone so a 50% increase. Not really ideal, even if in the real world the variability of conditions on the circuit, on the person and in the external influences would be such that you could never tell if someone would be killed by a particular current it would not be nice to think that you had not done all you are required to do as a minimum to save them.

[ElectriciansForums.net] RCBO timing too high

MDJ · Mar 9, 2014

In answer to the Troll here. Basically if you install a new distribution board and on completion the RCD or a RCBO in it fails the test then you have a care of duty to either replace the faulty unit or advise the customer the circuit it is connected to is not protected due to a fault. If there was a fault in the house 5 minutes after you left and a fire started or someone got electrocuted resulting in the death of someone the prosecuting barrister would have you for dinner in the court room. I can imagine his words now "you left the property unprotected and ignored the rules in BS7671" the jury would without doubt blame the electrician for the deaths because he would have known the RCD or RCBO was not compliant. I cannot actually confirm a law for you Troll but common sense tells us that if you change a board then you need to ensure it is functioning correctly, now be a good boy and go and run in front of a car.

Engineer54 · Mar 9, 2014

Originally Posted by southsea1201To a degree I am criticising the 30 mA RCD...it is way too high a current, but that is what we are saddled with. I know for all of us we make a serious attempt to comply with everything in the BGB. I certainly do, I also take extreme pride in my work, and can't compete with most people on an economic basis because my work takes longer. I get most of my work on recommendation because I work to a high standard and communicate all through the installation with my customer as to what I am doing and why I'm doing it. Probably fair to say that applies to all the veteran posters on this forum, because they are clearly interested in the subject matter rather than most people who go home and switch off from work.

Because we tend to protect multiple position circuits (eg light fittings/socket outlets etc) rather than single point outlet or equipment such as you would find in the States.

Trying to have a stable circuit running several bits of equipment, or several light fittings protected by a 6mA RCD, ...and more often than not you'll be on a hiding to nothing!! lol!!

As i stated, most socket GFCI protection is obtained by providing individual integral GFCI socket outlets in the States, rather than protecting the circuit as a whole as would generally be the case in UK/Europe.

30mA RCD's/RCBO's will provide a relatively high degree of personal protection, while also providing a good degree of nuisance free tripping when correctly installed into a system. If you feel a higher level of personal protection outweighs the very real possibility of nuisance tripping, then a 10mA alternative can always be substituted....

Geoffsd · Mar 9, 2014

Sorry if already said and I have missed it.

Isn't the fundamental flaw in southsea's argument that RCDs limit the fault current?

They don't so the time IS important

mechelec · Mar 9, 2014

MDJ said:
In answer to the Troll here. Basically if you install a new distribution board and on completion the RCD or a RCBO in it fails the test then you have a care of duty to either replace the faulty unit or advise the customer the circuit it is connected to is not protected due to a fault. If there was a fault in the house 5 minutes after you left and a fire started or someone got electrocuted resulting in the death of someone the prosecuting barrister would have you for dinner in the court room. I can imagine his words now "you left the property unprotected and ignored the rules in BS7671" the jury would without doubt blame the electrician for the deaths because he would have known the RCD or RCBO was not compliant. I cannot actually confirm a law for you Troll but common sense tells us that if you change a board then you need to ensure it is functioning correctly, now be a good boy and go and run in front of a car.

I am not a troll but you are entitled to your opinion as we all are. I am just showing again that you are keen to post big statements with no substance to back them up.

MDJ · Mar 9, 2014

I think my comment was quite justified, so you think fitting a new DB in someones house and then finding out when testing one of the RCBOs in it is faulty, and then leaving it anyhow is okay and legally acceptable then, and to add I did say could be considered illegal, and you are trolling and looking for trouble, my post was quite acceptable and no big statement, it was an opinion and a high court barrister may agree should someone get killed due to the faulty RCBO.

mechelec · Mar 9, 2014

MDJ said:
I think my comment was quite justified, so you think fitting a new DB in someones house and then finding out when testing one of the RCBOs in it is faulty, and then leaving it anyhow is okay and legally acceptable then, and to add I did say could be considered illegal, and you are trolling and looking for trouble, my post was quite acceptable and no big statement, it was an opinion and a high court barrister may agree should someone get killed due to the faulty RCBO.

I only asked which law it contravened. Not too much too ask after you said it would be illegal. This implied you knew which law was being broken.

i know you like to think of me as a troll but I have been a member here longer than most. That is not usual troll behavior is it ?

Dont take things so personally.

MDJ · Mar 9, 2014

mechelec said:
I only asked which law it contravened. Not too much too ask after you said it would be illegal. This implied you knew which law was being broken.

i know you like to think of me as a troll but I have been a member here longer than most. That is not usual troll behavior is it ?

Dont take things so personally.

I said it could be considered to be illegal

no okay it isn't, sorry

fair enough

polo1 · Mar 9, 2014

I'm interested in the point about RCD's/RCBO's failing the x5 test. If one accepts that <40ms must be achieved and subsequent replacements fail the x5 test, what do you do? What is people's experience - do you often find devices failing the x5 test?

Richard Burns · Mar 10, 2014

I normally find that RCDs pass both tests, however there are a few that fail (which is worrying since if they can fail from new when might they fail in use?).
I have just had a 30mA RCD test OK on a new install and then when it was actually under use it failed to reset even when the fault was clear.
I find that the trip times are much closer to the regulations limits with the modern RCDs with their electronic control, whereas I find older RCDs trip much faster but at the same time for both 1x and 5x.

If a replacement device fails on testing then I would suspect that there was a problem with the attached circuits and ensure that there was only the incomer attached and test then, if it still fails then it was two faulty units in sequence.

Hawkmoon · Mar 10, 2014

MDJ said:
I said it could be considered to be illegal

no okay it isn't, sorry

fair enough

Respect to you - manning up to apologise/admit error is woefully rare on this forum

southsea1201 · Mar 10, 2014

@ Richard Burns...Agree with you..I have never had a 61008 fail the test. Only 1 modern one failed for me and that was the RCBO I alluded to in this thread. It is interesting that 4293's always seem to fail for me...has anybody else found problems with them?

It's very interesting to note in the OSG (as much an advisory document as BGB) that when testing 4293's no advice is given to do a 5 times test...it only advises this for for 61008 and 61009. Again I could use this document in my defence.

Also 100% agree about testing at the board with circuits disconnected. On my test sheets I number my circuits as usual (say 1-10) but I have seperate lines headed RCD Module 1 and RCD Module 2 and treat these entirely seperately.

southsea1201 · Mar 10, 2014

Geoffsd said:
Sorry if already said and I have missed it.

Isn't the fundamental flaw in southsea's argument that RCDs limit the fault current?

They don't so the time IS important

But Geoff say you were in a "can't let go situation" and your body resistance at that moment in time would allow 22mA through you....time is a factor in electrocution...I'm sure multiple seconds at this very low current would kill you and the RCD wouldn't trip. When I got shocked by an RCD protected circuit I had to actually pull the wire off my finger with my other hand..I reckon I was stuck to it from anything between half and 1 second without the RCD tripping. It could have been longer...it is so difficult to perceive time when you are being shocked.

Geoffsd · Mar 10, 2014

I was referring to your accepting more than 40ms for >150mA and implying that RCDs limited the current (shock).

In your example above re: 22mA, (10 - 11kΩ ?), you are correct in that the RCD would not operate.
RCDs aren't a magic solution to everything.