RCDs

[timegoesby]

A neighbour has a Wickes CU with an RCD and two mcbs. One mcb supplies the electric oven. The oven had a small fault and the RCD tripped by-passing the circuit mcb. He also bought a cheap Indian made fan with a 3 pin plug with no fuse. I know this is illegal, so it must be an illegal import.

When having an RCD on a CU the fused plug seems redundant as the RCD cuts in to knock all the CU off. What is the point of a fused plug when the RCD overrides the lot? I can only see the need for the fuse if an appliance has no fault yet is drawing more current than what it should.

[dom]

RCD's detect differences in current between live and neutral wires, any difference may be due to current passing to earth by a route it shouldn't (e.g.through a person).

MCB's protect fixed wiring against overcurrents, and consequent fire risk. They're rated based on the thickness of fixed wiring.

Fuses in appliance mains plugs protect against the device drawing more current than it is designed for. It's sized on the devices max power needs (and so is the flexible cable on the appliance).

All 3 devices contribute to safety.

[Ron Lowe]

The 3 things you mention: RCD, MCB and plug fuse all do different things.

The RCD protects against even very small currents ( eg 30mA ) flowing to earth ( perhaps via some person touching a faulty appliance) rather than back out through the Neutral. This provides some protection against electric shock. A faulty appliance can trip an RCD without any over-current being present. This is what may have happened with the oven you describe.

An RCD, however, does not protect against over-current, which is a different type of fault. The MCB is an over-current device, to protect the wiring up as far as the socket. It is typically 32A for a ring circuit.

The plug fuse is to protect the downstream wiring ( flex ) to the appliance.

If a small appliance like a lamp with very thin cable developed an overcurrent fault, and the plug was not fused, then you rely on the 32Amp MCB. This is too big to adequately protect the skinny flex, which would overheat. Since the overcurrent fault will not trip the RCD, you have a hazardous situation.

[The Natural Philosopher]

MOST RCD's are actually combined with current detecting MCB type wwitches though..as in RCBO or main input RCD's..mine is a 63A rated one I think..

Indeed.

[Christian McArdle]

Is this a round pin plug? Most round pin plugs are unfused and run off a 15A or 5A circuit, so don't need to be. AIUI, India still uses makes widespread use of round pin plugs.

The RCD has nothing whatsoever in common with a plugtop fuse. They do different jobs in different ways.

The RCD does not under any interpretation of the word "override" a fused plug.

If it is drawing more current than it should, then the appliance can't be said to have "no fault". In fact, it can be said to be faulty.

Certainly, I would suggest that your apparent understanding of how protection systems work in UK style electrical installations seems very deficient. I would suggest ensuring that your electrical installation meets current regualtions by commissioning a periodic inspection and checking all your appliances to ensure that they have the correct plugtop fuse rating. Saying that you only need an RCD and apparently intend to use unfused plugs scares people.

Christian.

[Christian McArdle]

Nope. Although RCBOs incorporate such an overcurrent device, your main input RCD will not. The 63A rating does not indicate that it will cut off if the current substantially exceeds this. It just indicates that the device is suitable provided you don't expect the normal current through it to exceed the rating (unless there is a fault condition, where the number in the square is the figure, usually 6000A).

The only overcurrent/short circuit protection in most systems are the main incoming fuse, the circuit MCBs (or RCBOs) and any plugtop/FCU fuses. A standalone RCD in the consumer unit offers no such protection.

Christian.

[Guy King]

The message from [e-mail] contains these words:

Fuses are generally to protect the cable not the appliance. RCDs are supposed to be to protect people from shock from live things to earth.

[The Natural Philosopher]

Are you sure? In one place we worked the main RCD (63A) kept tripping.not from leakage either...we got a sparkie in, who looked at the cable tails to it and uprated it to 100A. That blew the 63A FUSES in the mains...so we got them uprated..serenity ruled..well until we tried to switch on with 120 PCS all connected. That blew it RCD wise :-D

Hmm. I am not so sure..but you may be right.

[Andy Wade]

Yes, but (with certain exceptions) it's not lawful to supply an appliance like this in the UK unless it's fitted with an approved fused

13 A plug.

[dom]

That sounds like a random "replace stuff until the problem goes away".

AIUI RCD's trip on current imbalance alone and the current rating is a safe max load through it, i.e. the physical capacity of the wiring inside it.

Otherwise what would be the point of a product called an RCBO, if RCD's did the same thing?

How do you know the RCD didn't trip on leakage?

[Ron Lowe]

That doesn't surprise me.

Many devices which have RFI filtering or surge suppression filtering will cause small amounts of earth leakage currents particularly at startup as part of their normal function.

Powering up 120 PCs together might well produce enough earth current to trip an RCD.

It might also generate enough inrush current to trip an MCB. It would depend on the MCB type.

I'd probably Not Do That.

[Christian McArdle]

Yes.

How do you know this?

The fuses see the same current as the RCD. If the fuse blows, then the MCB is hardly likely to have overcurrent protection, given that service fuses are very slow blow compared to a standard Type B MCB.

I should hope so. I suspect your issues are related to putting too many PCs (with earth based filtering). I seem to recall some guidance on the number of computers attached to a 30mA RCD. I can't remember the exact figure, but

15 rings a bell, and you have 8 times this number. Circuits to run these computers should preferably not be RCDed, or RCDed at 100mA with their own unshared RCD. They should also have a high integrity earth configuration.

Christian.

[Sparks]

120 PC's all on one RCD - no wonder it was tripping!

Switch mode power supplies all leak a little to earth through their supressors, I would be suprised if 120 PC didn't trip it!

[David Hansen]

On Wed, 26 Jul 2006 14:11:39 +0100 someone who may be The Natural Philosopher wrote this:-

Most is a bold claim. I very much doubt if it is correct.

That is the steady state current it is rated for. Pass double or quadruple that current through the RCD and it will still not operate, though it might melt after a while at quadruple the current.

[The Natural Philosopher]

Because when replaced with a Bigger One, of the same RCD rating, it Did Not?

[The Natural Philosopher]

We ended up OK...on earth leakage..just..had to uprate the main fuse and main switch tho...earth leakage would trip if we tried to switch em ALL on at once, so after ever power cut it was a yell of 'switch the PCS OFF'.

[John Rumm]

Not normally the case IME. The 63A rating you see on many RCDs is just the nominal maximum current carrying capacity. Stick a couple of hundred amp through one for a susstained period and it may "blow", but only if you include melting into a big sticky mess as an allowable definition of "blow".

RCBOs however do include MCB style overcurrent protection. Perhaps you had one of those (although these are only commonly available in ratings up to 40A).

[Christian McArdle]

You'll probably find your quiescent leakage is somewhere between 15mA and

30mA, where the RCD's operation is undefined.

Christian.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Natural Philosopher saying something like:

[Pete C]

I reckon some RCDs have current transformers that don't balance high frequency RFI and big current transients very well. Eg they trip on things like blowing light bulbs.

Maybe some makers are aware, so newer ones or different brands are less prone to this problem.

cheers, Pete.