Up-front RCD requirement -TT system.

[ling-electrical]

Hi,
I have recently com across a job in which I have been asked to carry out some upgrading and various remedial work. It is a TT system and the tails from the meter to the consumer unit are run about 5 meters, under the floor, via a 60 a KMF located next to the meter.

The consumer unit a 17th edition so there is no problem there as all final circuits are covered by 30mA RCDs.

My question is do I still need to install an upfront RCD for additional protection on the tails as the are below the floor and in a partition?

Usually with the consumer unit next to the meter this would be pointless. Its a substantial cost to install an S-type RCD if its not really necessary and I cant seem to get a straight answer on the subject, seems to be a bit of a grey area!

Any help is appreciated!

Thanks
J

 

[mechelec]

Is the KMF an insulated enclosure or steel? If steel the RCD a must. With regard to the tails I would say at 5m there is a chance of potential fault to earth so again RCD is a given.

 

[ling-electrical]

Thanks. Its an insulated KMF, me only concern was the additional protection on the supply tails.

cheers

 

[Octopus]

Thanks. Its an insulated KMF, me only concern was the additional protection on the supply tails.

cheers

I would venture to suggest that if the CU is plastic, and you are not touching the CU or the tails I would leave well alone. May be a note on the cert.

 

[mechelec]

I would venture to suggest that if the CU is plastic, and you are not touching the CU or the tails I would leave well alone. May be a note on the cert.

I would be concerned as the tails run under the the floor so are not visible and could potentially get damaged by nails, vermin etc. This could end up leaving a fault to earth of insufficient magnitude to clear the protective device and cause a fire. RCD is the way to go.

 

[newbec]

My question is do I still need to install an upfront RCD for additional protection on the tails as the are below the floor and in a partition?

Its a substantial cost to install an S-type RCD if its not really necessary and I cant seem to get a straight answer on the subject, seems to be a bit of a grey area!

J

Be aware that if you install an S-Type RCD you will not be fulfilling any requirements for additional protection. Additional protection is afforded by an RCD with a rated residual current of not more than 30mA and must disconnect within 40mS at 5xRated residual current. An S-Type RCD will not comply.

 

[SPARTYKUS]

Be aware that if you install an S-Type RCD you will not be fulfilling any requirements for additional protection. Additional protection is afforded by an RCD with a rated residual current of not more than 30mA and must disconnect within 40mS at 5xRated residual current. An S-Type RCD will not comply.

Hmmm.

might stop that fire though......

 

[newbec]

Hmmm.

might stop that fire though......

Who mentioned fire?
Additional protection of the tails was the OPs questioning of the need for an RCD

 

[Engineer54]

Be aware that if you install an S-Type RCD you will not be fulfilling any requirements for additional protection. Additional protection is afforded by an RCD with a rated residual current of not more than 30mA and must disconnect within 40mS at 5xRated residual current. Only when they are working correctly!!! An S-Type RCD will not comply.

If were talking about a typical UK 200 ohm Ra TT system, then you'd be a bloody fool not providing a 100mA S type up front RCD device. They may not comply as additional protection, but they WILL back up a failed downstream 30mA RCD!! Relying on single 30mA RCD's each side of a 17th ed board, each covering several circuits, is not something i would be happy with on a TT system if it was my house and my family!!

Some of you need to see the amount of duff RCD devices that are sent back to the manufacturer on a regular basis on our projects. You might then change your mind on the reliability placed on RCD devices!!

 

[SPARTYKUS]

Who mentioned fire?
Additional protection of the tails was the OPs questioning of the need for an RCD

See above post. I mentioned fire

 

[newbec]

They may not comply as additional protection

Which is all I was pointing out. As this is the description given by the OP as to what protection he was attempting to afford with the use of the RCD. I'm not commenting on any other reason or possible scenario, just simply the facts as given.

 

[SPARTYKUS]

Newbec

comprendez ;-)


I read it differently, He seemed more concerned with rodent damage to tails. Which might cause a short and therefore a fire. I assume he wasn't worried about the rat going into VF mg_smile: