RCDs in a tt system

[big willie]

I recently installed a split board wylex consumer unit in a tt system that had been converted from a tns system by the provider about three years ago.The old system consisted of a consumer unit with old mini breakers only.When the system was changed to a tt the provider installed a 100Ma rcd external to the consumer unit as an on and off switch. Now that I have installed a new fully compliant with the 17 edition fuse board protected by two 30Ma rcds do I still need the 100Ma external RCD. EFL at source is 19.7 OHMS.

 

[Bobster]

It's not doing any harm and a back up in case one of the RCDs fail.

It can also be used as an isolator for the new cu.

Why was the system changed from tns to tt?

 

[Guest55]

technically no but i'd still keep it there as a failsafe and point of isolation.

 

[Guest55]

Why was the system changed from tns to tt?

dno's simply cant be arsed repairing thier own sheath earths these days , that costs money whereas leaving the customer to provide his own earthing costs f. all.

 

[tony mc]

Make sure the 100 mA RCD is S type or any fault on any side of the CU can take out the tails RCD !

 

[Bobster]

dno's simply cant be arsed repairing thier own sheath earths these days , that costs money whereas leaving the customer to provide his own earthing costs f. all.

Thought so.

 

[MDJ]

I recently installed a split board wylex consumer unit in a tt system that had been converted from a tns system by the provider about three years ago.The old system consisted of a consumer unit with old mini breakers only.When the system was changed to a tt the provider installed a 100Ma rcd external to the consumer unit as an on and off switch. Now that I have installed a new fully compliant with the 17 edition fuse board protected by two 30Ma rcds do I still need the 100Ma external RCD. EFL at source is 19.7 OHMS.

No. .

 

[Engineer54]

I recently installed a split board wylex consumer unit in a tt system that had been converted from a tns system by the provider about three years ago.The old system consisted of a consumer unit with old mini breakers only.When the system was changed to a tt the provider installed a 100Ma rcd external to the consumer unit as an on and off switch. Now that I have installed a new fully compliant with the 17 edition fuse board protected by two 30Ma rcds do I still need the 100Ma external RCD. EFL at source is 19.7 OHMS.

They wouldn't of got away with that little move if it had been my installation. Fine if it was a temporary measure, but no way as a permanent bodge job!! They are duty bound to ''maintain'' a previously DNO provided TN earthing provision!!

As for your query, it would be prudent to change that straight 100mA RCD for an S type 100/300mA RCD. Why, because you're totally relying on a single RCD (either side of the split CU) for your earth fault protection, on a TT system without it. And unfortunately RCD's are just not that reliable to be trusted, especially over extended periods of time. As others here have stated, it will also provide a useful means of isolation to the main CU.

 

[big willie]

I think it was because the sweated earth connection to the incoming main had become detached and they could not get the EFL low enough with a strap on. And further more some dno's dont allow strap ons anymore

 

[Engineer54]

I think it was because the sweated earth connection to the incoming main had become detached and they could not get the EFL low enough with a strap on. And further more some dno's dont allow strap ons anymore

Of course they do, what they don't, or shouldn't allow is BS 956 clamps to be attached to PILC cables. But Hepworth and constant force spring clamps are pretty much in standard use by all DNO's for upgrading or reattaching main earthing conductor sizes to PILC cables...

 

[shagbite]

I think it was because the sweated earth connection to the incoming main had become detached and they could not get the EFL low enough with a strap on. And further more some dno's dont allow strap ons anymore

surely, someone called 'big willie' would never have to bother with a strap-on.

 

[stitch]

I wondered when the strap on jokes were going to start.