cu advice gladly needed ......

[ez2own]

Right gentlemen need to ask your advice , thanks in advance ..

Going to change cu to a 15 way with split rcd to bring up to 17th/part p , original board is 14 way with rcd protected on main switch only .
My only concern is small 4 way cu in garage ( if i rcd protect will possibly trip main board in house if anything went pear shaped , would i be best to leave small cu in garage with rcd protection , but place into house cu with rcd not connected to garage supply.
only supply garage has external is 2 lights on a motion sensor .or would no rcd in garage be a non comply ????

Thanks again ...

 

[Flying Ninja]

Hi, You can get time delay rcd to get over that. Alternately depending on the type of supply you have and weather or not the garage in connected to the house you should have a TT system in the garage. Regardless you could TT the garage and not connect the earth from the house to the garage supply but connect at house to ensure the supply cable is protected.

 

[GLENNSPARK]

If using an armoured for the garage supply then you don`t need an rcd as it has an earthed metallic protection....so the best way is to run the garage armoured feed off an MCB without RCD and to RCD it at the garage board....this will give you the discrimination you need...........

 

[ez2own]

Thanks glenspark swa to garage , so feed into mcb without rcd protection , and would rcd main swith on garage main switch do the job ???

Mega thanks fro ya help ..

 

[nickblake]

If its a 15 way Hi integrety CU and the cable is installed in a way that it does not require RCD protection then use an MCB from the unprotected side of the board to protect the garage CDU with the RCD ,this way if the RCD trips then it save running to the house , if it does require RCD protection you can eith use an RCBO at the house and replace the main RCD isolator to a normal isolator in the garage or as been mentioned use a time delayed RCBO in the main house

 

[Engineer54]

Hi, You can get time delay rcd to get over that. Alternately depending on the type of supply you have and weather or not the garage in connected to the house you should have a TT system in the garage. Regardless you could TT the garage and not connect the earth from the house to the garage supply but connect at house to ensure the supply cable is protected.

Yet another TT system fanatic!!

 

[nickblake]

Yet another TT system fanatic!!

LOL agree ,why TT the garage , you still have the same problem with RCD discrimination and supply cable protection

 

[Flying Ninja]

You Dont Have the problem as you will not be exporting the earth from the house. And the cable will be protected via RCD in the house??

 

[GLENNSPARK]

You Dont Have the problem as you will not be exporting the earth from the house. And the cable will be protected via RCD in the house??

And whats wrong with an "exported earth"?....

 

[Knobhead]

So which one trips first?

View attachment 8229

 

[Flying Ninja]

I am not saying there is a problem with exporting the earth or getting into a debate about exporting PME etc. I merely aswered a question on how to solve the problem of two RCD's tripping and posted what i thought to be a couple of solutions.

 

[GLENNSPARK]

Would it not depend on the slight sensitivity variations of the individual rcd (even though they are both 30mA) tony? Wouldn`t even ambient tempreture and age of the protective device play a part in this...not to mention the impedence of the fault path.....

 

[GLENNSPARK]

I am not saying there is a problem with exporting the earth or getting into a debate about exporting PME etc. I merely aswered a question on how to solve the problem of two RCD's tripping and posted what i thought to be a couple of solutions.

As i posted before run the supply cable (armoured) from an MCB being fed directly from the incomer at the house board for 5 second disconnection time and then RCD it at the garage board as i said before this will give you the discrimination you need without getting into costly terretory (s type RCDs)..............

 

[Knobhead]

This is in reply to Glen.

One thing you have to take in to account is there will be a slight leakage from the house circuits added to any leakage from the garage.

So say you have 10mA leakage in the house (PC’s, fridges Etc.) it’s effective range is now 20mA, where as the garage remains at 30mA.

The question remains the same, which trips first?

 

[Engineer54]

So which one trips first?

View attachment 8229

Seems you are the only one that knows, that your not relying on a ground to trip an RCD. having two separate ground points will make little to no difference at all ...lol!! But your house connection point on your drawing is in the wrong place. lol!!

Glennspark,

The ONLY way to ensure discrimination between similar rated RCDs, is by a time delay.
Iv'e known 30mA and 500mA having no discrimination between them...