Rcd- mcb

[davidcaulfield]

Hi guys in installing a 16mm submain with a 10mm earth and I'm going to install a 80amp time delay RCD with a 63amp MCB to protect it. Some one said I could just use a 63amp RCD to protect it? Never done this before and wouldn't think you could.

Also should I just use a 63amp RCD with a 63amp MCB

 

[trev]

Nowhere near enough info.

 

[danzor]

What's the sub main? T+E or SWA?

If T+E put a time delay on it, then 30mA at the new board

 

[Darkwood]

Are you suggesting having no overload protection on your sub-main supply?? More info please.

 

[telectrix]

does it need RCD protection?

 

[Guitarist]

Agree, not enough info. Very unlikely a time-delayed RCD will be of any use to you anyway.

 

[DPG]

Are you thinking that the 63A RCD will trip on overload of >63A ?? Daz

 

[telectrix]

it's a common misconception that RCDs and Main switches ( e.g. BS 60947-3 ) will trip if the rated current is exceeded. keeep telling guys that the A rating of a switch/RCD is just the load it's designed to carry.

 

[DPG]

A spark carrying out work for people would know this though?? Daz

 

[Jay Sparks]

A spark carrying out work for people would know this though?? Daz

You'd be suprised!!!!

 

[telectrix]

you'd think so, but how many sparks don't understand the principles of bonding? it's not done " in case water/gas pipes become live", but it's to ensure that they rise to an equipotential with the fault so that the p.d. is as near to zero as possible. not easy to explain.

 

[davidcaulfield]

Sorry guys for the quick writing

I am going to protect the 16mm T&E submain with a time delay RCD and a 63amp MCB. The guy I was working for buys all the bits and said I should jut use a 63amp RCD. I have never heard this and didn't think they protected against overload but don't want to argue with him if there a chance he's right (which I doubt) just think I would ask on here if all these years I have been doing it wrong

The time delay RCD will be feeding a 2 bed granny flat which will have a 17th edition board.

 

[davidcaulfield]

The main question I was asking is if I'm using a 63amp MCB, what rating time delay RCD would you normally use... 63amp , 80amp , 100amp??

They only had 80amp in stock so I said this will do?

 

[DPG]

It's the max current the RCD can work at, not what it trips at. Daz

 

[Darkwood]

You seem to be in confusion because someone has contradicted what you know.... RCD's dont protect against overload and the Amp rating is the max that the unit can safely carry like a 15amp connector will carry up to 15amps so in essence of this fitting a higher rated will be fine as would using 30amp connectors to replace 15amps even though the load would not exceed 15amps.

We questioned why you think you need a time delayed rcd in the first place?
We lack alot of info'

 

[davidcaulfield]

The time delay RCD protect the T&E submain when running surface in the garage, underground , then up a dot and dab wall to the consumer unit.

I no it doesn't need RCD protection as its a distribution circuit but surely prevention is better than cure?

I just thought if some one ever damaged it or drills it then the RCD could trip out in 0.04 second? Potentially saving a life?

Or am I getting it wrong as I've never used one before???

 

[Darkwood]

Regardless of it been a sub-mains supply it would still be subject to the same regulations that all other circuits are regarding 30mA protection unless its was say an SWA .... so i query the fact to it having time delay surely fitting a standard front end 30mA rcd is the way to go and all the board is then protected.... if you didn't want the tripping to drop out the supply to numerous circuits you should have wired it in swa then rcbo'd the circuits.

If you dont have freezers etc in the garage this isn't really an issue and front end protection is fine.

How is the cable protected underground is it fully enclosed throughout in conduit or you just run it through plastic ducting? Also what depth?

 

[GLENNSPARK]

it's a common misconception that RCDs and Main switches ( e.g. BS 60947-3 ) will trip if the rated current is exceeded. keeep telling guys that the A rating of a switch/RCD is just the load it's designed to carry.

yep....and it would be funny if it wasn`t just so poor....

 

[DPG]

I wonder how many installations out there have circuits protected by an RCD only, ie. where the spark is thinking that the device is giving overload protection as well as the RCD function. There could be a lot of dangerous situations arising from this. Daz

 

[GLENNSPARK]

to the O/P:
the lads need to know the earthing arrangements for a start...

what type of cable you intent to use for this sub...

whether or not theres any extranious bottom end...

what the max demand is going to be...