Karl had a question about protecting a distribution circuit in a TT system. The tech team at the NICEIC answered the call.
This Q & A is one of the thousands posted in our Technical Expertise area and answered on a daily basis by our Voltimum Experts.
Question:
In a TT system where the meter is located in a detached garage and the consumer unit is within the property; how would I protect, and achieve a disconnection time of 1sec, the 60A distribution circuit supplying the consumer unit which contains 30mA RCBO’s without discrimination?
The existing sub-main (or distribution circuit) is at present protected by a 60A rewirable fuse and is enclosed within a plastic pipe which is buried under the concrete path before it enters the premises.
This circuit, (I can only assume) enters the property (bungalow) either buried in the wall or via the cavity!
I do not wish to renew the distribution circuit for obvious reasons: cost etc.
Regards
Karl Schäfer
Answer:
In a TT installation regulation 531.4.1 requires that at the origin of a TT installation it shall be protected by either a single RCD placed at the origin unless the part of the installation and the device complies with the requirements for protection by the use of class II equipment or equivalent insulation as given in section 412.
Where an RCD is installed at the origin of the installation and further RCD’s are installed in series regulation 531.2.9 requires discrimination in their operation where necessary to prevent danger.
To achieve discrimination one option would be to install a time delay type ‘S’ device such a 100ma device.
Where a time delayed RCD is used refer to Table 3A in Appendix 3 for time/current performance