Dimmers and MCB's

G'day,
Somewhere I recall reading that dimmers require CB's on the 240V input side and the output side to comply with regs. My sparky says "male cow excrement" just one on the input side. He has done a number of cBus installs. I've also seen a few too and don't recall any CB's on the output side and as that would make 9 CB's per dimmer I'm sure I would have noticed that.

What are the regs?
What do most of you do?

tia
build

 

Your sparky is right. You don't need a CB on the load side of the dimmer. The CB's are to protect the cable from over current. Having additional CB's downstream from the Dimmer unit is not a requirement. It would be like putting a CB after each conventional dimmer in your house.

Cheers

 

That's exactly what I thought. But, page 16 of the CDBus Hardware Installation manual under 4.4 Protecting CDBus Loads shows dimmers require CB's.

Conformist, can you catagorically state that the regs don't require this?

 

As long as the CB providing power to the unit (load supply) is correctly rated for the cables connected between the unit and the loads, there is no need for additional CB's on the individual loads. The whole issue is protecting against worst case fault current. Don't for a millisecond think that CB's on the load side is going to provide any sort of protection for equipment channels for things like short circuits etc. The triac output stage will blow long before the worls's smallest CB will trip. If you want there is nothing stopping you putting CB's on the load side. It will cost lots and take up lots of DB space.

 

Yep...

You are looking at a diagram which shows the 'line' side of the connection (going to the unit) which has a Circuit Breaker. These connections usually come from the same circuit breaker. The load side of the dimmer doesn't have a breaker.

Cheers

 

Thanks

Thanks Gents,
So the only reason for a CB on the load side is to protect the Dimmer and it won't do that anyway.

Is that Correct?

cheers,
build

 

That's correct. The reaction time of the breaker is way slower than the sizzle time of the SCR/triac

cheers

 

I think you'll find that the wiring diagram (showing MCBs on the output side) that you refer to only applies to conformation with IEC (european) wiring standards. For example Page 8 of the L5504DU2 installation leaflet states :

As far as I am aware, in Australia, this is not required as wiring doesn't need to conform to this IEC standard. The electrician is the one taking responsibility for the safety of the installation, so let them advise you on this.

For the record, here is the wording of the titles :

IEC 60669-2-1-2002
SWITCHES FOR HOUSEHOLD AND SIMILAR FIXED-ELECTRICAL INSTALLATIONS PART 2-1: PARTICULAR REQUIREMENTS - ELECTRONIC SWITCHES

IEC 60669-2-2-2002
SWITCHES FOR HOUSEHOLD AND SIMILAR FIXED ELECTRICAL INSTALLATIONS - PART 2: PARTICULAR REQUIREMENTS - SECTION 2: REMOTE-CONTROL SWITCHES (RCS)

 

Just addding - the use of an MCB on the output (to conform to that IES standard) is relating to safety, not to protecting the C-Bus devices.