Consumer Unit Advice - need a rethink

OK, I've had CBUS installed for 6 months now (did it myself), and now I need to re-look at how it's wired.

I've had a few issues, basically tripping the RCD in the main CU. The outside lights have tripped and so do a couple of my extractors in the bathrooms/en-suites (strangely only when turning off).

So, I have 2 x Dimmers and 1 x Relay unit (need another relay soon tho).

My Hager CU has lights on the right (now disconnected), and mains on the left. All mains on the left is through the panel's RCD.

From this mains side, is a 32Amp MCB, and I take the Live and Neutral from the CU to the first CBUS Panel, which goes into a Twin MCB (so I can isolate Live and Neutral totally).

From this , the neutrals and earth in those cabinets are all commoned.

The Live feed now goes to 3 MCB's, a 10Amp for each dimmer, and a 20Amp for the relay.

Each Live Load from the dimmers and the relay goes though another MCB (mostly 1Amps units for example).

Now, as Im getting this problem tripping back, mebe I should be doing something different here.

After some suggestions and expert advice please !

Thanks

G.

 

home wiring experts

another expert doing his own wiring !
not hard to work out what you have done if you have an electrical background
maybe you should let someone who knows what they are doing to look at it for you

 

First off Gareth, the dual MCB on the input is a no-no, you simply cant have a protective device in the neutral, and I'm not convinced an MCB is acceptable as an isolator.

But thats not the problem you're facing.

Nuisance tripping RCDs are one of the hardest problems to sort out, but the first approach is to only supply via RCD that that is required to be supplied by RCD, or that it is sensible to supply by RCD. I cant remeber what the UK regs say these days, but you need to run a "split load" arrangement whereby you have RCD and non-RCD protected circuits.

 

Terminology

OK, I think I need to clarify my terminology !

The Dual MCB is in fact a Dual Pole Isolator - one of these puppies - Dual Pole Isolator

I had spoken to a few chaps at the electrical shop who suggested the split load arrangement.

Im assuming I should then replace my MCB's on devices that individually need RCD protection (using an MCBO) such as outside lights.

G.

 

Turns out you are allowed to double pole MCB a single phase circuit (reg 530-01-02), though most folks would think it an "unusual" thing to do.

Yes, using combined MCB/RCD is an excellent if expensive solution.

Be aware that for circuits that dim you should have a MCB that is "pulsating current" rated.

 

Copied

I basically visited a chap who had had his whole system installed by an expert, and I've basically copied the way it was done - except I didnt see the part from the CU.

So all my MCB's etc are exactly the same. Even the cabinets.

So I'm fairly comfortable on that front - just the main feed to the CBUS units.

G.

 

Ummm - am I missing something here ?

The tripping of a RCD unit is caused by a unbalanced load - there is a different amount of current running down the active as coming back the neutral (in Australia that would normally be > 30mA difference to cause it to trip)

You said that the External and Extractor Fan circuits seemed to be causing the tripping. What do both these have in common ?

MOISTURE ! I am guessing that moisture is getting into the luminaires, fans or wiring and causing a small leak between active or neutral to earth. That leak is causing the RCD to trip.

Before you worry about the board or the Cbus, I reckon you should look at the loads. The only way of trouble-shooting it is to isolate all the loads until you determine which circuit / device is causing the tripping. Then fix it

I don't know if in my ignorance I am missing the point, but as far as I'm concerned a RCD is there to protect humans coming in contact with dangerous currents - it's not there to protect the electrical devices.

Disconnecting a circuit / load from a RCD because it's causing the RCD to trip kind of defeats the purpose of using one, doesn't it ? It also might be illegal (in Australia, lighting circuits must be RCD protected)

John

PS: Guess what time of year it is in the UK - coming into winter. Might there be more moisture about in winter than summer, and that is what's causing the problem to "re-appear" after months of semming to be OK ?

I know that in Sydney it hasn't rained for many many months. Recently there was finally some rain, and we heard from many clients who had outside lights that had started to trip the RCDs !

 

Yeah, same regs as we now have in NZ.

In the UK, RCD protection regs (unless they've changed recently) are based on rules rather than mantra. In NZ (and I guess Australia) the regs result in whole house RCDs being fitted, so everything goes off when it trips. Thats a daft reg on a number of fronts, and hopefully in a few years our collective authorities will realise this, and appreciate that RCDs are excellent tools when used in an appropriate manner, which means (in domestic terms) selectively.

Mind you, given that a standard lamp with 0.5mm wire is breakered at 20A, I'd worry about that in terms of how many house fires are started that way...

 

Tripping

JohnC

Yes, there's a (newish) in-line extractor for one of the en-suites, and this has tripped 5/6 times in 6 months. Only has tripped when turning off. It's in the loft space - so mebe worth me replacing it uner warranty.

The outside lights tripped twice (but Ive since replaced them)

G.

 

Gareth,

Just to complete the picture of what my expert (hi Ben) did at the CU.

All on the non-RCD side. Two existing MCBs (upstairs and downstairs - fairly standard), plus two new ones so four in all. Each MCB feeds a single C-Bus output unit (and you'll remember that I have four of them).

AS an aside.... Later addition was contactors on the four outside lights. That was because the switch-on surge of the lamps was tripping the [individual] MCBs on the outputs. I had learned on here that outside lamps are quite prone to frying C-Bus relays, so didn't want to just increase the MCB ratings. Hence the C-Bus relay now drives a contactor and the contactor drive the lamp.

Nick

 

RCD's

Nick

So no RCD/MCBO's on your outside lights ?

G.

 

Nope, they are well up on the wall out of touching reach (I think having to get the ladder out would be sufficient reminder to isolate them properly).

I have never lived anywhere where any of the lights (inside or outside) have been RCD protected.

Don't know the regs, but I trust Ben !

Nick

 

In Australia, all lights in *NEW* installations must be RCD protected. So, if you put in a new circuit, it's gotta be RCD'd.

I have been in the Lighting Industry for over 20 years. I doubt very much that there's any truth in the rumours that Exterior Lights "draw more current", "trip RCDs" or "fry Cbus".

Those all sound to me like bad installation practice that is causing the problems.

I know that I would be mighty concerned if a particular circuit caused protection devices of any form to trip ! And I don't think I'd be circumventing the protection, but rather I'd look to solbving the problem.

However the comments about using multiple RCD/MCB's are very valid. They are NOT that much more expensive anyway... I have been buying them off eBay and a single (dual pole) 10A or 15A unit costs between $15 and $25 brand new. Considering that you'll pay between $5 and $10 for a plain RCD, that extra money is NOTHING in the overall scheme of things.

Cheers, John

 

A 300 Watt halogen light running on 240 volts takes 1.25 Amps. The switch-on surge trips a 2 Amp quick blow MCB every time. I have proved this on the bench, no C-Bus, just a simple circuit.

Cheers

 

Hi Nick
When the lamp is first turned on the filament is cold and the resistance is much lower so theres an inrush current.....
For instance i have a 100w halogen lamp who,s cold resistance is 40 ohms ..this equates to 6amps when first turned on.....Thats why normally a bulb blows when turned on as its filament is cold and its resistance is much lower....

HTH
Frank

 

Fair enough, Nick - but that hasn't got much to do with RCDs tripping (the subject of this thread). The situation you describe applies to ALL incandescent loads, most fluorescents, all discharge lamps (Mercury, Metal Halide, etc) and almost all motors, They draw more current at startup than they do to "run".

Keep in mind that it's a short-duration "spike" of current, the maximum is immediately upon switch-on and then it decreases very quickly as the filament warms. Therefore, it is not a sustained "overload" and any decent electrical device (switch, dimmer, relay, contactor) will be designed to accomodate that surge.

My comment was made because an inference was made in a previous post that Exterior Loads were more "difficult" on C-bus than interior loads...

The *location* of the load is not the issue, it's understanding the type of load and what is required to drive it and to protect the circuit. For protection, there are different "response curves" available for MCB's. If the "quick blow" MCB's are tripping, then perhaps they're not the correct type or rating for your intended usage.

If you are using Cbus relays, then each relay contact is rated at 10A. If the only reason you are adding a downstream MCB is to protect the relay, then the rating of that MCB should be 10A, shouldn't it ?

Alternatively, take a look at the Tripping Characteristics of the MCBs that you are using. For example, taking that 2A breaker tripping... A C-curve Clipsal MCB should take about 1 sec to trip at 6 times the rated MCB current (ie: 12A on a 2A breaker). At 3 times (6A on a 2A breaker) that same C-curve RCD should take 5-20 seconds to trip.

There's a big difference between a Quick-Blow MCB and a C-bus relay. If the C-bus contacts are rated 10A Incandescent (ie: L55xxRVF), then that means that they are rated to handle the STARTING CURRENT of a 10A worth of incandescent load.

On a 240V supply you can theoretically connect 2400 watts of incandescent load onto each channel and not blow the relay. If a C-bus relay burns out when loaded with less than 10A, then it's a warranty issue - I doubt very much that Clipsal would rate their relays at 10A if they couldn't handle 10A worth of load connected to them !

Cheers, John