RCD discrimination

[Revved Up Sparky]

Hi guys, happy new year to you all !.

I'm doing a mains change next week on a TT installation. The meter tails enter a 100mA RCD from which a 16mm T&E submain runs to the consumer unit.

I want to fit a dual RCD consumer unit in line with 17th edition regs.

I also want the change the main RCD from 100mA to 30mA in order to ensure the submain feeder also has the 30mA RCD protection required.

BUT !.... this creates a discrimination problem. If any circuit on either side of the consumer unit has an earth fault then the main RCD will probably trip before either of the dual RCDs on the consumer unit trip.

It would be very difficult to extend all the final circuits and position the consumer unit where the cutout is so what should I do ? How can I provide 30mA RCD protection to both submain cable and final circuits whilst also having a correctly functioning dual RCD consumer unit where the main RCD feeding the submain will not trip before either of the dual RCDs ?

Many thanks in advance.

 

[Badger]

if the sub mains in in trunking or containment ie surface installed then no need to protect tails

 

[Revved Up Sparky]

if the sub mains in in trunking or containment ie surface installed then no need to protect tails

Even on a TT installation Badger ? I thought everything had to have 30mA protection on a TT.

 

[Badger]

not for tails if installed on the surface as far as im aware. as long as there is 30mA protecting all circuits then afaik that is within regs, only time rcd has to be provided to tails is if they are less than 50mm deep in the wall/fabric of building/property.

 

[Andy™]

Even on a TT installation Badger ? I thought everything had to have 30mA protection on a TT.

not always. you could use 100mA TD for submain, providing its surface etc

 

[Larnacaman]

Hi guys, happy new year to you all !.I'm doing a mains change next week on a TT installation. The meter tails enter a 100mA RCD from which a 16mm T&E submain runs to the consumer unit.

I want to fit a dual RCD consumer unit in line with 17th edition regs.

I also want the change the main RCD from 100mA to 30mA in order to ensure the submain feeder also has the 30mA RCD protection required.

BUT !.... this creates a discrimination problem. If any circuit on either side of the consumer unit has an earth fault then the main RCD will probably trip before either of the dual RCDs on the consumer unit trip.

It would be very difficult to extend all the final circuits and position the consumer unit where the cutout is so what should I do ? How can I provide 30mA RCD protection to both submain cable and final circuits whilst also having a correctly functioning dual RCD consumer unit where the main RCD feeding the submain will not trip before either of the dual RCDs ?

Many thanks in advance.

The chances are, even the 100mA RCD will trip, when either of the split load RCDs trip. If you want to do this properly, change the straight 100mA RCD for a time delayed S Type RCD. This will ensure cascade discrimination.

 

[SPECIAL LOCATION]

Even on a TT installation Badger ? I thought everything had to have 30mA protection on a TT.

Have a quick glance at the examples on On-Site-Guide pages 22 -26.

Application of RCD's

As t'others have said Don't forget "time delayed" needed for discrimination.

Guinness

 

[Larnacaman]

You DON'T need 30mA protection up front, 100mA is more than suitable for this purpose...

 

[PC Electrics]

As already mentioned 100mA S type for distribution circuit. You could also use 2 core swa for some extra protection and might even look neater.

 

[Chr!s]

You treat the meter tails/distribution cable the same as any other, there's no exemption for tails.