RCDs

[Ross]

You have an interesting style of debate, Ross. I shall answer your points in the same order. .

Well I didn’t know I had an interesting style of debate, but thanks for pointing that out.

The upstream MCB will not protect a dimmer from a fault condition downstream. By the time the upstream MCB trips due to a fault downstream, the dimmer channel could already have been damaged. By fitting a downstream MCB (as well as the upstream one, of course), it should protect the channel from over-current and short-circuit fault conditions. It also allows easy isolation of a particular dimmer channel to carry out work on. As you are aware, C-Bus dimmer channels are not isolated from the mains feed even in the off state.

Perhaps you should revisit my last post. I stated “The point is the upstream RCD will see the short before the downstream RCD every single time.” You seem to be confused with the use of an RCD over an MCB, and the combinations thereof.
Furthermore what you are endorsing is just complete rubbish. As for fault currents and their dark secrets, we shall leave for another time. Suffice to say any quality MCB RCD with a fault current rating of 6kA or better WILL protect the dimmer in the majority of cases, that is the fact weather you choose to accept it or not. There is no guarantee that a dimmer will survive any short circuit however, sanity says use a quality product and get on with it. The point on the additional benefit of having groups individually isolated is complete rubbish. Turn the line side breaker off. Problem solved. What is the issue with that?

There is no written reference to my statement except that it is endorsed by the CIS UK Technical Manager, Steve Gordon. Also, on a number of CIS quotations to customers that we have copies of, the lack of any downstream channel circuit protection was explicitly stated. Steve has advised that any damage caused by inadequate circuit protection on both sides of the dimmer is not covered by CIS standard warranty. He further confirmed that damaged dimmers are currently not repairable by CIS UK for the foreseeable future.

Like I said before I can appreciate the problems with getting equipment repaired. As for Steve Gordon’s take on things, I would suggest he get up to speed with the CIS documentation as should you. Perhaps Steve would like to discuss this with us. I would welcome the opportunity. As for no written documentation from CIS, (the company that manufacture the products) supporting your installation methods, need I say more.

If a customer has 8 circuits to control with a C-Bus 8-channel dimmer, I hope you are not suggesting that he or she should install an additional dimmer module as redundancy.

No, I am suggesting, if possible use another dimmer that will switch things like bathrooms and laundry and external lighting and leaving 2 spare in lieu of using a relay. Or a combination of both or taking a “chance” as the rest of the world does and use quality equipment and deliver a quality installation. As I’ve said before my experience with product failure is rare at best. I will state however that the majority of all C-Bus installation problems stem from incompetent installers playing with things they have little understanding of.

A protective MCB for the purpose costs less than 3 British Pounds per channel and a suitable DIN rail enclosure costs around 35 Pounds (all plus VAT) if additional rail space is required.

Let’s see. One of my company’s current projects is a residence on the beach and has 4 off 8 channel dimmers and 2 off 4 channel dimmers (as well as 6 relays). That’s a total of 40 dimming groups to protect and using your method is hardly justifiable, nor sensible. Geez I could toss in an 8 channel dimmer cheaper than going down that road.

May be it is not worth mentioning the quick circuit isolation feature of having downstream MCBs for electricians and people that are electrically competent. For the rest of us, poking around inside a live DIN rail enclosure with a screwdriver rearranging potentially live wires is hardly a simple or desirable activity. Swapping channels and reprogramming the C-Bus dimmer unit, for most people, involves a chargeable visit by the installer/integrator whereas an MCB will provide the protection automatically and conveniently.

Please, this is just complete rubbish. If you have no idea what you are doing then leave well enough alone. Trained systems integrators and endorsed and qualified installers are the only people who should touch a C-Bus installation in any case. That is the whole point, if you aren’t qualified then don’t touch. You may want to check that one out with CIS as well.

The C-Bus dimmers, as you have stated, are good units but they are not infallible. Dimmer channels do get damaged under certain fault conditions and the occurrence is higher that you are experiencing. For the sake of a few extra Pounds per channel, I maintain that fitting appropriate dimmer channel circuit protection is a sensible and cost-effective precaution.

And I say that this is complete tripe. It would appear we shall have to agree to disagree. I could go on but I feel I am kicking a dead horse.

[Ross]

OK - so, if I go with the protection on either side of the dimmer, what size MCB should I be looking at for the load?

Each channel has a 1A rating and the smallest standard-size MCB I have seen is a 3A. This would therefore allow 3 times the rated current flow before tripping.

Ideas? :roll:

Save your money Ric. You would be just adding components that are not required. Read my post below for clarification.

[Ross]

Hi Ric
Had a quick look in the Clipsal Catalogue and they do 1amp circuit breakers 6ka or 10ka .. here in Oz.....So i would imagine Merlin Gerin or others would manufacture 1 amp ones as well.... I still would recommend hrc fuses maybe using fused terminals ;-)
Frank

Frank,
A 1 amp 6ka circuit breaker has a trade price of 12.72
A 1 amp 10ka circuit breaker has a trade price of 27.59

Besides the cost of breakers and switch boards to house them, what do you gain? Nothing. A fully loaded group may well trip a 1 amp breaker with no fault at all.

This is just not a valid consideration.

[Darbyweb]

Interesting topic

I've arrived a bit late in the proceeding but'll add my 2p's worth anyway.

Firstly there seems to be a bit of confusion with terminology.

RCD - Residual Current Device.
MCB - Miniature Circuit Breaker.

In the UK we do not need to have RCD protection on lighting. IEE Regs require protection for all outlets which could be deemed for use outside the equipotential zone, i.e. Garden.

Standard practice in the UK is to protect all sockets throughout, leaving Cooker, Appliance Ring, Heating, and lighting circuits off the RCD.

I would certainly class a standard, quality, CBus installation in the UK to include outgoing protection of all Dimmer ways.

As correctly detailed a 10amp MCB would be suitable to feed each dimmer, and then on the outging side fit single 1a Type B MCB's.

These are easily obtainable from good wholesalers.

Part number as used by ourselves.

Hager NB101 1amp 10kA.
Also available in 0.5, 2, 4, 6 and onwards.

Good practice would always dictate that the local circuit has an immediate means of isolation that is clear to the end user.

To say that they would need to isolate 8 circuits to work on a single light would be a little unfair, as would suggesting a screwdriver and connector as another means of isolation would also be a little foolhardy and down right dangerous - one dead customer later :cry:

Likewise telling them to remove the offending 8 channel dimmer - send it away to Clipsal and leave them with 7 more non functioning circuits would certainly not be on..

From a customers point of view (he's always right remember) to blame him for damaging his own equipment, whilst correct, would probably end your relationship with him all for the sake of £2.50 per channel.

We have recently taken over an install where the client had used a 'qualified' CBus installer. He had not recommended inline MCB's - customer has already lost 2 x channels and is not a happy bunny !!

From my own perspective, in a rush to do work at home i stuck in an additional 8 channel dimmer without fitting MCB's - cut through the wrong cable, and bang goes 1 channel - local MCB goes out (10amp) as does ELCB protection (1960's overhead supply)

As a professional installer, we cannot do anything less the 100% - perhaps the Blame/Claim culture has not hit Aus yet, but anything that looks like a shortcut and consequently affects what has been installed, will come back and bite you in the arse.

Thanks for your time



Dean.


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[FrankMcAlinden]

Hi Dean

I would certainly class a standard, quality, CBus installation in the UK to include outgoing protection of all Dimmer ways.

This appears to be UK specific standard practice from information i have received from others...Going on the cost of the cbus stuff im not surprized.
For Example 15 years ago when plc,s were very expensive we had to incorporate a mcb for each output which went to a field device ie solenoid. This was a requirement from the customer...Today mcb,s would only be required for groups of outputs .....

If you have a dead short on a dimmer load i dont believe the rcd/mcb on the line side of the dimmer would be quick enough to protect the triac ....To give the dimmer a good chance of survival i would prefer to use a quick acting fuse and if i had the room in my electrical box i would have installed terminal fuses...
Frank

[Ric]

I've certainly started somewhat of a debate here!

I'm still having difficulty finding 1A MCBs though. The samllest MCB I seem to be able to find is 3A and the smallest RCD is 16A. The name of a good supplier would be helpful.

[Darbyweb]

Ric

Any Newey & Eyre or Edmundsons wholesaler branch will be able to obtain the MCB as the part number and manufacturer i listed.

Its not a debate - its facts


Dean.

[Ric]

OK - I've found somewhere to buy the MCBs relatively cheaply.

1A MCBs for less than £3 + VAT
8A MCBs for less than £7 + VAT

Check out www.thefusecompany.co.uk.

[Darbyweb]

Ric

You dont need an 8amp MCB - 10amp is correct rating and will be available off the shelf for a lot less.

Also refering to your earlier post you dont need/want an RCD protecting the lighting circuits.

£3 + VAT doesnt sound too bad.




Dean.

[Ric]

You dont need an 8amp MCB - 10amp is correct rating and will be available off the shelf for a lot less.

According to the Clipsal documentation you should provide 8A of protection. The 8A MCBs are more expensive but I'd rather use the recommended spec at a slightly higher cost than run the chance of damaging a £450 dimmer unit which would not be under warranty because it had been incorrectly installed.