Puzzled By The RCD In My Workshop

[niall Y]

The lights went out in my workshop, this morning , while I was using an Earlex wallpaper-steamer. No problem, I unplugged it, got a torch, and went over to the small consumer unit to switch things back on - only to find nothing had tripped. Then indoors to check the main consumer unit, to find that the RCD had tripped here.
This used to happen in my old workshop, where power came into my workshop from a neighbouring one, it was always the the fuse or RCD in the neighbours workshop that went first, rather than the one in mine. Is this just Murphy's Law in action, or is there a reason for this?
Both the RCDs are rated at 30 ma , so ; is one slightly more sensitive than the other? I'm curious..........

 

[guineafowl21]

They do vary in trip level and timing, and the one further upstream will experience a tiny bit more reactive leakage from the extra cable length.

You’re meant to step the sensitivity of protective devices progressively down as you reach the outlet. That way, it’ll be the nearest thing that trips first. Used to be called discrimination.

 

[Spectric]

This is a case of poor design, you have an issue with discrimination which is there to ensure the protective device nearest the fault disconnects rather than the upstream device protecting the actual supply. If this is wrong then it makes the protective devices in the workshop pointless as they are not doing there job, discrimination can be based upon time, current or both. From what you say the supply to your workshop has been taken off the main board and is therefore supplied via the RCD, you don't need an RCD in the supply to your workshop as you should have RCD protection in the workshop board in the form of an RCD or preferably RCBO's. The best and safest solution to your problem is to get an electrical tradesman in, not a domestic installer as this could be above his paygrade.

 

[niall Y]

Thanks for the replies - very interesting. So, this is a known 'thing' catered for by 'discrimination'. And 'Mr Murphy' has nothing to do with it.

This then leads to the naive question "why aren.t all main consumer units fitted with a different RCD as standard ? " That way an RCD further down the line, in, for example an extension lead, would have a chance to react first.

In my previous workshop this did present a problem, as I did not always have access to the main RCD, but with the present one it isn't. Unless, of course I'm missing something here?

 

[Spectric]

This then leads to the naive question "why aren.t all main consumer units fitted with a different RCD as standard

Because they have been designed for the domestic circuits within the property, hanging a supply off for a workshop impacts that design. I can recal when we had boards with no RCD protection, then a single RCD that caused all sorts of issues and then split boards with two RCD's which helped but now we have boards with no RCD's and each circuit is protected individually by an RCBO which is a combined device for residual current inbalance and short circuits but for some reason the likes of screwfix still sell the old boards, must have brought to much stock. For a workshop supply you just need an upstream device to protect the supply cable to your workshop and a board in your workshop for distribution to the circuits where some form of RCD protection is provided.

 

[HamsterJam]

Used to be called discrimination.

Or graded protection.

 

[Spectric]

I think it is supposed to be called selectivity now as it was thought to be discriminatory to call it discrimination.

 

[clogs]

just to add
My garage next to the house has an RCD device for protection.....
It was always tripping out when using the small welder....
the house electrical system is 15 years old.....
change the RCD for exactly the same and works great now......
I guess they can age......?
What say u Spectric......

 

[Spectric]

RCD's do degrade over time but the BIGGEST issue is that people just think they are well protected and don't test the RCD using the inbuilt test feature and they can fail to work. From an electrical testing aspect we test them as found, at several currents at both 0 and 180° phase to check disconnection times and have often found devices that have initially failed to operate. Using the manual test button will often trip them and they then will pass the electrical test and you have to advise the owner to manualy test more regular. Much easier with industrial as it is a different ball game.

 

[John Brown]

RCD's do degrade over time but the BIGGEST issue is that people just think they are well protected and don't test the RCD using the inbuilt test feature and they can fail to work. From an electrical testing aspect we test them as found, at several currents at both 0 and 180° phase to check disconnection times and have often found devices that have initially failed to operate. Using the manual test button will often trip them and they then will pass the electrical test and you have to advise the owner to manualy test more regular. Much easier with industrial as it is a different ball game.

What part ages, just out of curiosity?

 

[Spectric]

They just don't last forever and you find they get to a point where they either cause nuisance tripping or are no longer giving protection even though they are essentially a simple device, I blame the mechanical aspect myself. Now with so much technology being used and we have higher Dc components there is the added problems that this impacts the characteristics of the RCD and hence why the regs changed with the 18th .

 

[MikeJhn]

I sometimes found that operating a sticking RCD a few times improved its performance and gave better readings, have I read that RCD's don't have to be tested x5, I'm retired and not kept up with the latest regulations.

Don't worry found this: Changes to RCD testing in BS 7671:2018+A2:2022. seems its the 180deg that is not needed anymore, will have to tell my Fluke on automatic not to bother in future.

 

[fleyh]

Can someone recommend a plug-in/socket type RCD with faster reaction time than the ones used in the consumer unit?

I am having similar problem when a device connected to a plug-in RCD causes main consumer unit RCD to shut off everything. Both consumer unit and socket RCDs are 30mA

This one is useless, as it does not prevent main consumer unit RCD from tripping. I have tested the socket RCD and it reported no issues.

Consumer Unit always trips before the plug-in RCD.

 

[Spectric]

Yes they believe they have simplified the testing procedure whilst increasing the amount of words in the regs !

The logic behind this is that if a manufacturer has tested something prior to sale then it cannot fail in service which is like saying your new washing machine has been tested and will not go wrong !

There wording

" It was noted that those testing RCDs in the field would find it difficult to carry out the variety of tests permitted and may not know which of the available tests would be applicable to a given make or type of RCD.


It was therefore agreed that the testing procedure could be greatly simplified and safety would remain unaffected since RCDs are, in any event, tested extensively by manufacturers prior to sale.
"

They must have forgotten about electrical engineers and think we only have domestic installers.

When it comes to safety, testing and inspection is essential and simplifying a process because you don't think people are capable is a road to no where because now we have so many more types of RCD's with different characteristics then just showing it works is probably pointless if the wrong type has been installed.

The IET summary.

RCD's do degrade over time

What part ages, just out of curiosity?

The last line of the summary highlights this issue.

seems its the 180deg that is not needed anymore

Yes according to the 18th but with some types of RCD it may highlight an issue and if your tester does it automatically then just let it do it.

With selection of devices you can now only use type AC RCDs to protect circuits where there are NO Dc components, otherwise type A RCDs have to be used. These will only operate under fault conditions where the Dc components do not exceed 6mA and is now why just having a split board with two RCD's is problematic because with so much newer technology imposing Dc on the CPC you can easily exceed this value and using a higher number of RCBO's is often required. The Dc issue is that Dc will saturate a transformer like the ferrite core used in RCD's and so any inbalance in line and CPC goes undetected so the device fails to trip.


I have heard that

 

[Spectric]

Can someone recommend a plug-in/socket type RCD with faster reaction time than the ones used in the consumer unit?

Maybe the device is working correctly and you have an issue elsewhere, what are the devices causing the issue?

 

[fleyh]

Maybe the device is working correctly and you have an issue elsewhere, what are the devices causing the issue?

It is a dishwasher.
I believe somewhere inside that dishwasher there is a broken seal which lets some vapour out and, about once a month, it trips the electrics mid wash cycle (no issues during drying part).
After it had tripped the electrics - If we let the dishwasher sit there for a few of hours it then works fine again for another month or so.
There is no water visible anywhere inside or around the dishwasher.
No other power hungry devices are usually active at the time of the issue.

I have moved the dishwasher to another wall socket - did not help.
When we run a kettle with 3 other power hungry devices at the same time in the nearby wall sockets - no issues. So I have to conclude that it is a dishwasher specific problem and not a house electrics issue.

Replacing this dishwasher or calling in an electrician (only to tell me that dishwasher is an issue) is not economical at the moment, so I figured I could plug the dishwasher via a plug-in/socket RCD, but that did not help with the main consumer unit.
I realise that this is a somewhat adventurous approach, but that's the current situation.

 

[Spectric]

So your main RCD is detecting the inbalance to your dishwasher and tripping which is good, no one is going to get hurt. Your problem is the local plugin device is not sensitive enough, I can see you want local disconnection and not the main RCD tripping as that is inconvenient but your issue is with discrimination (sensitivity) and you could try

https://www.cef.co.uk/catalogue/pro...Vid_tCh38cwADEAQYASABEgJG8_D_BwE&gclsrc=aw.ds
but no guarantee's . This trips with 10mA residual current and IN THEORY will trip before the 30 mA device.