TT System and RCD

[Shaunoffshotgun]

Ive seen mentioned a couple of times now about a TT System ( i think i understand this, am i right in saying its an earth rod in the ground
system ? ) having to be RCD protected.

Can somebody please explain in laymans terms why this is.

 

[Qedelec]

This is because of protection.
As the earth is supplied via the mass of earth (into which the rod is inserted) it can vary and rise to unacceptable levels.
It is desirable to limit touch voltage to 50V so an RCD of 100mA and ohms law give a max resistance of 50/0.1 = 500 ohms

In practice the resistence should not rise above 200ohms

 

[Adam_151]

TT is where the earth is derived by an electrode sunk into the earth as opposed to from the suppliers cables, this results in a high ELFI, typically about 120ohms (as opposed to a max or 0.7 or 0.35 for the other two methods) this allows a max current of 230/120 = 1.9A to flow, which means that if an earth fault developed, its not going to be able to trip a breaker, so we protect agaisnt earth faults using RCDs instead, typically the main rcd in a TT system is 100ma, the RCD is sized so that earth leakages will trip the RCD before they raise the touch voltage above 50V (25V in case of farms, etc), multiplying the resistance of the earth path by the operating current of the RCD will give this voltage, and it can be seen that for example if you used a 30ma rcd, you have a max value of 1666ohms, though it is rescommended that it should be kept below 200 ohms, as anything above that value is deemed to be unstable

 

[Shaunoffshotgun]

Thanks for that lads

So would i be right in saying that the main earthing on a TT system is not as reliable as it is on other systems so the extra (RCD) protection on all the circuits is always needed ?

 

[securespark]

RCD protection on TT should be 100ma TD incomer plus 30ma on the other side.

If the CU is metallic, the rcd should be external to the CU, if not, the CU should be insulated.

 

[Pensdown]

If the CU is metallic, the rcd should be external to the CU, if not, the CU should be insulated.

To save me looking it up, what reg does that refer to?

 

[securespark]

Not sure, but basically what it's saying is you cannot have a metallic unit incorporating an rcd on TT because there is no protection on the CU itself whereas if the RCD is on the tails, then the metal CU is protected by it.

 

[Adam_151]

IIRC, its also acceptable to use a special incomming gland kit that eliminates the possibilty of an earth fault being able to occur prior to the RCD?

 

[comms]

Try regulation 531-04-01 for metal clad cu's on a TT system & rcd's at origin.
There was a good write up about this in pro electrician a few years ago.................
Back again found it it was march 2004 for those that are interested or sad enough to want a back copy.

 

[Pensdown]

Thanks guys