RCBOs: A or AC, B Curve or C Curve

[EventMan]

We operate in a multi-use space, which has a small number of permanently hung LED fixtures, as well as some of our dimmer-fed tungsten generics.

The existing LEDs are plugged in to a circuit with sockets that are not easily accessible to most of the volunteers who run the venue, and so are consequently left live all the time.

The committee has agreed to get a couple of spare ways in the distro board to enable isolation of these fixtures, as well as some new LED fixtures to be bought soon.

These circuits would supply - almost exclusively - the LED fixtures. The only exceptions would be some DMX splitters and a wireless DMX transmitter.

Should we spec' type A or AC RCBOs and should they be B Curve or C Curve?

Thanks for reading!

[themadhippy]

The regs have recently changed,so unless its fixed equipment were theirs no chance of any DC  its got to be  type A,and personally id suggest a  C curve to avoid nuisance tripping  from inrush  currents

.

[p.k.roberts]

I was watching this on the ETC YouTube channel the other day; it may answer most of your questions as well as some you haven't thought of yet ...

 

[Tomo]

Type AC RCDs are not suitable for any of the loads you've listed - or indeed most loads people are likely to encounter these days.

For breakers, you need C or D curve as the inrush currents for LED fixtures are very high.

With LED fixtures, in most cases you will hit the leakage current or inrush current limits long before troubling the overcurrent rating.
- The video PK links to above goes into that in a bit more detail!

Edited May 4, 2022 by Tomo

[Simon Lewis]

5 hours ago, themadhippy said:

The regs have recently changed,so unless its fixed equipment were theirs no chance of any DC  its got to be  type A,and personally id suggest a  C curve to avoid nuisance tripping  from inrush  currents

.

We are not meant to say "nuisance" anymore - it gives the impression that the RCD is being deliberately vexatious! "Unwanted" is the correct term 😉 

Using type C does help with inrush current (5-10 times rated current vs the B curve's 3-5 times rated current) but the earth fault loop impedance of these circuits needs to be suitable as well.

Edited May 4, 2022 by Simon Lewis

[Jevans]

Depending on the fixture type you may get less fixtures on a circuit than you might like. I know from bitter experience a C10 curve RCBO won't reliably take the inrush current of more than 2-3 ETC Source4 Series 2 Lustrs for example, as they have a very high first half cycle inrush (107A on the spec sheet!!) Other fixtures may have inrush limiting components built in.  

It's not entirely clear from your description if you're intending to use the protective device (RCBO) itself as the means of functional switching, but I'd recommend specifying a separate switch or contactor to use for the functional switching rather than regularly operating the protective device - if anything, this will mostly prevent volunteers accidentally switching off the wrong circuit.  

As themadhippy says, Amendment 2 to the 18th edition of BS7671 which came out a few weeks ago has effectively discontinued the use of type AC RCDs, as they are now only permitted for fixed equipment which is known to not use DC components. 

[sunray]

11 hours ago, Jevans said:

Depending on the fixture type you may get less fixtures on a circuit than you might like. I know from bitter experience a C10 curve RCBO won't reliably take the inrush current of more than 2-3 ETC Source4 Series 2 Lustrs for example, as they have a very high first half cycle inrush (107A on the spec sheet!!) Other fixtures may have inrush limiting components built in.

On a slightly different vein we installed eight strobe lights in a nightclub mid 90's which claimed to be something like 250W each.

15A type 2 MCB, DP switch and 4 double sockets. They worked as master slave with a remote speed control.

Our solution to stop the tripping was to change the switch to start stop buttons, a couple of contactors a timer and an additional MCB which had to be 60A type 4 to start all 8 together then drop back to the 15A.

The next time was similar except we used a resister to limit start-up current before the contactor took over.

[Stuart91]

33 minutes ago, sunray said:

 we installed eight strobe lights in a nightclub

I remember one lighting company who had a "special" distro for their strobes, populated entirely with D-curve breakers...

[EventMan]

Thanks, all

That concurs with what I remember from a seminar by James Eade at a PLASA just after the 18th Edition came out (or was due out) - i.e. Type A C Curves. And the video was very useful!

My doubts were raised by a retired electrician who not only said that RCBO/RCDs weren't needed, but that B Curves would do (old dogs and new tricks come to mind!).

Thankfully, that was elsewhere and he is not involved in this install. I can now price up the parts and get a spark booked.

Yes, I had intended to have 20amp DP switches installed for isolation, but I'm not sure whether to have these with neon indicators because I know they can make some domestic LED fittings glow. I don't know if this would be an issue. I would prefer them as an easy visual check that everything is off.

I'm planning on about 10 fixtures per circuit. But, having watched that video, I might suggest installing a separate distro just for the LEDs, with multiple circuits. I'll have to see how it goes.

Thanks again!

Edited May 5, 2022 by EventMan

[sunray]

8 minutes ago, EventMan said:

Thanks, all

That concurs with what I remember from a seminar by James Eade at a PLASA just after the 18th Edition came out (or was due out) - i.e. Type A C Curves. And the video was very useful!

My doubts were raised by a retired electrician who not only said that RCBO/RCDs weren't needed, but that B Curves would do (old dogs and new tricks come to mind!).

Thankfully, that was elsewhere and he is not involved in this install. I can now price up the parts and get a spark booked.

Yes, I had intended to have 20amp DP switches installed for isolation, but I'm not sure whether to have these with neon indicators because I know they can make some domestic LED fittings glow. I don't know if this would be an issue. I would prefer them as an easy visual check that everything is off.

I'm planning on about 10 fixtures per circuit. But, having watched that video, I might suggest installing a separate distro just for the LEDs, with multiple circuits. I'll have to see how it goes.

Thanks again!

We did condider a ring final using 4 or 6mm Cable and a 63A MCB and switch as all of the strobes are on their own 13A plug.

[Jevans]

4 hours ago, EventMan said:

My doubts were raised by a retired electrician who not only said that RCBO/RCDs weren't needed, but that B Curves would do (old dogs and new tricks come to mind!).

The last few editions of BS7671 make it very difficult to not install RCD/RCBO protection on any 13A or small IEC 60309 socket outlet. I don't have the new exact wording in front of me, but my recollection is a written risk assessment is required to be filed with the electrical installation certificate, and the outlets are required to be under the control of a 'skilled' person.  

4 hours ago, sunray said:

We did condider a ring final using 4 or 6mm Cable and a 63A MCB and switch as all of the strobes are on their own 13A plug.

That wouldn't work - the terminals on 13A sockets themselves aren't rated to 63A. Generally they're also not rated to have multiple 6mm cables in the terminals. 

[themadhippy]

Quote

- the terminals on 13A sockets themselves aren't rated to 63A. Generally they're also not rated to have multiple 6mm cables in the terminals. 

And it wouldn't meet the definition of an A1 ring

[sunray]

3 hours ago, Jevans said:

The last few editions of BS7671 make it very difficult to not install RCD/RCBO protection on any 13A or small IEC 60309 socket outlet. I don't have the new exact wording in front of me, but my recollection is a written risk assessment is required to be filed with the electrical installation certificate, and the outlets are required to be under the control of a 'skilled' person.  

That wouldn't work - the terminals on 13A sockets themselves aren't rated to 63A. Generally they're also not rated to have multiple 6mm cables in the terminals. 

 

2 hours ago, themadhippy said:

And it wouldn't meet the definition of an A1 ring

I did say we considered it.

Edited May 5, 2022 by sunray

[EventMan]

Regarding all of the above, has anyone successfully installed Inrush Current Limiters in their circuits?

Eaton did do one, but it's now discontinued - although it looks similar to one by PROLED, which provides AC peak current at 165 A for 20 ms / 800 A for 200 μs and AC continuous current at 16 A https://www.proled.com/en-DE/products/controls-power-supplies-accessories/cables-accessories/inrush-current-limiter/