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To RCD/not RCD

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To RCD/not RCD

Postby UNMUTE » Wed Feb 01, 2012 1:32 pm

Hello Everyone!

This is my first post on here so apologies if this question has already been discussed.

In the interests of safety, I know it is good practice to use an RCD (RCCB). I'm wondering if others who do live sound/PA regularly use an RCD with their setup?

Does it lead to nuisance tripping due to multiple ground paths etc.?
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Re: To RCD/not RCD

Postby Hugh Robjohns » Wed Feb 01, 2012 2:16 pm

Modern RCDs trip when the current on the live side is out of balance with the current on the neutral side, on the assumption that the 'missing' current has gone to earth somewhere... possible through an unfortunate person!

I've never had problem tripping due to multiple ground paths.

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Re: To RCD/not RCD

Postby UNMUTE » Wed Feb 01, 2012 2:26 pm

Thanks for your reply Hugh.

An RCD will usually trip between 18-30mA.

If power is being supplied via a single RCD to a typical rack setup with a mixer (perhaps an amp. too), each item with its own supply filtering from live/neutral to earth, is this likely to exceed ~18mA?
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Re: To RCD/not RCD

Postby Hugh Robjohns » Wed Feb 01, 2012 2:31 pm

I hope not! I routinely run a complete DAW-based multitrack recording and monitoring chain off a single RCD without any problems at all.

Obviously, there is a practical limit as to how much can run off a single trip because there will always be some leakage current, but it's a surprising amount. it would probably be a good idea to run separate racks off separate trips.

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Re: To RCD/not RCD

Postby UNMUTE » Wed Feb 01, 2012 2:42 pm

Hugh Robjohns wrote:it would probably be a good idea to run separate racks off separate trips

Just to be clear, when you say trips, do you mean MCBs (Mini Circuit Breakers i.e fuses) or individual RCDs?
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Re: To RCD/not RCD

Postby Hugh Robjohns » Wed Feb 01, 2012 4:00 pm

I meant separate RCDs

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Re: To RCD/not RCD

Postby dmills » Wed Feb 01, 2012 5:12 pm

RCD always!

I have a backline distro system that has an RCD per socket (10mA) to try to ensure that there is some discrimination, each outlet on the main distro has a 30mA RCD, and the 63A socket the whole mess plugs into is usually on a 100mA Time delay RCD.

With competent modern ones, there is pretty much no such thing as a nuisance trip, if it goes there is something wrong.

The only time I have seen a leakage problem take out an RCD was with a whole pile of LED lighting, which was somewhat notorious for it, we put an isolating transformer in the way, problem solved.

If you set up the distro so that there is no easy way to bypass the things then you do not get tempted when the pressure is on.

Expensive, but what price a life?
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Re: To RCD/not RCD

Postby planetnine » Thu Feb 02, 2012 11:22 pm

The only times I've known "nuisance" tripping is with too small an RCD value on too large a supply network -particularly if outdoors and damp are brought into the equation. ie the ampages and distribution is all ok, and all devices seem to be ok individually, but the residuall just tots up to too much...

At one small festival site I work at, a 63A generator feed was split into two 32A feeds (stage and a underground distribution feed to 16A "posts" for stallholders lamps and fridges). Both feeds were protected by 30mA trips, but the generator trip was also 30mA. Neither 32A feed RCD would trip (and I tested them at various leakages with a trip tester to check they were good), but during the wet, the common RCD would trip occasionally.

I suspect general dampness in cables and connectors and lighting fittings totting up to trip-level. I have had measureable insulation readings on halogen lamp fittings (ie <99Mohm, still passing, but noticeable) while PAT testing -no visible moisture on disassembly, but a 5 min operation dries them out enough to go >99Mohm. The underground cable checked ok on insulation on its own, and I went round testing the dozens of stalls and their appliances (groan).

They solved the festival issue with separate generators. By adding a leakage of 5mA did get the field network trip to spring when used in a fully connected state, so it was very close to the knuckle. Measuring residual current, you could see it varying according to the weather!

I had a similar problem at a Bob Geldof gig where the 63A 3ph supply was on a 30mA trip. The TM got shirty after the third outage in soundcheck (as you'd expect) and we had to bolt a dist box onto some busbars to split the rig across more than one RCD. Simply too much gear across one trip. None of the sound Co.s dist trips would go, but the residual totted up to trip the main one. After splitting, not a problem all night.

Im a little distant from the regs these days Mr Mills, are there guidelines for this sort of thing now? Ampages, numbers of appliances and recommended trip values?

Problem is, it's often temp setups, and too many people put stuff together without knowing anything about electricity basics as it is; they think the ampage numbers are mumbo-jumbo we quote to spoil their fun!

>
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Re: To RCD/not RCD

Postby dmills » Thu Feb 02, 2012 11:48 pm

BS7909 for temporary event power is the document you want, it in tern makes heavy reference to BS7671 as you would expect.

Also, turning up BOTH the trip point and the time delay on the generator is the approved cure (Either one on its own you may still get tripping).

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Re: To RCD/not RCD

Postby planetnine » Fri Feb 03, 2012 12:22 am

If I had a choice, I'd do that, but most of the "little" gen sets don't have adjustable trips

For the bigger gigs, hired minions tend to do all that stuff in advance

Who'd be an SE at a homegrown festival in a British summer..?

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