100ma RCD

[sparkie-leeds]

I have been to a job this morning, the outside carpark lights had a fault on them, i have rectified the fault. The system is TT with 100ma rcd incomer protecting the installation.

The problem is that if there is a fault on the lighting circuit throughout the night it takes the main 100ma rcd switch out shutting all power down.

If i put the outside lighting circuit on a 30ma rcbo would this over come the problem of tripping the main switch out if there is a fault and ONLY trip the 30ma rcbo for the outside lighting?

 

[polo1]

Time delayed 100ma RCD incomer with 30ma rcbo on the lighting circuit?

Regards.

 

[sparkie-leeds]

will this over come the problem

 

[polo1]

Think so, as your problem would otherwise be achieving discrimination. But, might be worth a wait to see if anyone has other ideas?

Regards.

 

[Knobhead]

Don’t you think finding the fault is more important?
Yes you can chop and change the RCD’s, but the faults still there.
Fitting a time delayed RCD affects the whole installation and certainly doesn’t improve the safety.

 

[telectrix]

only sensible way i see to overcome this is to split the tails and feed the outside lights separately through RCD and MCB . this would not compromise the main installation's protection.

 

[polo1]

Don’t you think finding the fault is more important?
Yes you can chop and change the RCD’s, but the faults still there.
Fitting a time delayed RCD affects the whole installation and certainly doesn’t improve the safety.

Err, fault has been found! Would never suggest RCD/RCBO to help cover a fault. Fair point about S type affecting the whole installation and I should have spent more time when posting to caution about this. Do you think adequate discrimination will be achieved on a straight 100ma RCD?
Regards

 

[Engineer54]

will this over come the problem

It will, but at the detriment to the rest of the installation. As now you will have a time delayed up front RCD protecting the rest of the installation, not good at all ...lol!!! Now if you replaced all the existing MCBs for RCBOs then the whole installations protection will have been improved to a much higher level, which can only be a good thing, on a UK TT system!! lol!!

 

[telectrix]

It will, but at the detriment to the rest of the installation. As now you will have a time delayed up front RCD protecting the rest of the installation, not good at all ...lol!!! Now if you replaced all the existing MCBs for RCBOs then the whole installations protection will have been improved to a much higher level, which can only be a good thing, on a UK TT system!! lol!!

that, of course is the expensive alternative, but as most customers are tightarses, mine's cheaper.

edit: to conform with current regs. of course, a 30mA RCD (or RCBOs) should be installed to all circuits on a TT system.

 

[Engineer54]

. Do you think adequate discrimination will be achieved on a straight 100ma RCD? Regards

NO, Absolutly Not!! It needs to be a time delayed 100mA unit!!!

 

[sparkie-leeds]

I agree, but with the 30ma rcbo installed for the outside lighting circuit, if there is a future fault on the outside lighting circuit surely it would take the 30ma rcbo out for that circuit instead of taking the 100ma main switch out?

 

[PC Electrics]

NO

NO

NO

How many more times do we have to point out to ELECTRICIANS that two RCDs in parallel across the same supply DO NOT PROVIDE DISRCIMINATION.

RCD discrimination can only be achieved with two parallel RCDs if the upstream one is an S type (time delayed unit).

You will have to split out the car park circuit and put it on its own RCD.

Don't 'they' teach this stuff?

 

[telectrix]

not necessarily. could take out the 100mA, esp if the 30mA had not been regularly tested, or was a bit slow. a simile wasa here yesterday on my dragon vivarium. the lamp blew, took out the 3A fuse in it's plug, the 3A fuse in the stat. plug, and the 16A MCB in the DB.

 

[Engineer54]

I agree, but with the 30ma rcbo installed for the outside lighting circuit, if there is a future fault on the outside lighting circuit surely it would take the 30ma rcbo out for that circuit instead of taking the 100ma main switch out?

Only if the 100mA RCD is time delayed!! ....Most leakage faults will be above 100mA, and with all circuits being monitored, it could well be getting on towards that figure... Even the surge from a tripping 30mA device may set the 100mA unit off...

 

[telectrix]

NO

NO

NO

How many more times do we have to point out to ELECTRICIANS that two RCDs in parallel across the same supply DO NOT PROVIDE DISRCIMINATION.

RCD discrimination can only be achieved with two parallel RCDs if the upstream one is an S type (time delayed unit).

You will have to split out the car park circuit and put it on its own RCD.

Don't 'they' teach this stuff?

think you meant to say "series"

 

[ackbarthestar]

I found that you need to test each 100mA 'S' RCD with each 30mA RCD/RCBO combination to find out the precise time they disconnect. We know at 1 times they should operate between 130ms to 500ms and at 5 times they should operate between 40ms and 150ms but it is not always the case that discrimination is can be achieved.

The 4293 RCCBs have a lower front end of 200ms and maximum of between 300 and 400ms and often only trip near the maximum time allowed.

 

[Brightspark2]

think you meant to say "series"

ha ha how ironic...dont they teach that stuff? ha