New consumer unit on TT Supply - whats the regs.

**whitebeetle** · 30-06-2009

Hi , Got to replace consumer unit on my property (to use as my part p assesment !) At the moment its TT Supply (with earth rod) and feeds to the consumer unit through a 100mA RCD. It is only a small cottage and has the following circuits - Garage supply,cooker,2 x socket ring mains,central heating boiler and 1 lighting circuit.

In order to please the part p assesors whats the best way to play it after ensuring the Ze to be less than 200 ohms (i'm working on this !). Is the 100mA rcd ok or do I need to dropdown to 30mA ?

Can anyone tell me / explain the best /cheapest way to fit new consumer unit to meet current regs on a small domestic TT Supply ?

Thanks - Chris.

**electricAl** · 30-06-2009

What we offered here... Earth fault loop impedance reading on TT System in domestic premises

...still stands, Chris. The Regs aint changed in 4 days!

Is there a specific question you don`t feel has been answered satisfactorily?

Re: the Q`s in this post; as already stated you`ll need 30mA RCD`s for any bathroom/shower room circuits + socket outlets + for ccts with cables installed in walls at less than 50mm. As that`s likely gonna include your lighting circuits (& you`ll need 2 or more) you`ll require 2 or more RCD devices (could alternatively be RCCB or RCB0) so as to provide discrimination between devices.

Basically, nowadays TT`s are much the same as TN`s in requirements for CU`s

The 100mA is required only for ccts not requiring the `Additional Protection` of 30mA RCDs, would have to be of the `S` type (delayed) for discrimination of devices, & may be done away with entirely if you can meet the applicable criteria listed at table 41.1 of the Regs. (then you adopt TN values)

Seriously mate, you`ll have to read up on the Regs to get thru your assessment, so have a peak now

On the Ze front, while the official figure is min 200ohms, i`m pretty sure the NIC require 100 (which i`d always look to get even tho` i aint with them) so if it`s an NICe assessment you`ll need <100ohms

Hope that clarifies it further for you

**willosparky** · 15-11-2009

like Al said. as long as your Ze is less than 100 ohms and you fit a dual Rcd board (1 80 amp and one 63 amp rcd, both 30mA) you will be fine. if i was you id put the, garage, cooker and one ring on the 1st rcd, the other ring, heating and lights on the other. The MK sentry split load board is a gd one to use.

**not clever** · 15-11-2009

Seen as you only have 6 circuits why not put in rcbo's & if you use an insulated cu do away with the 100mA rcd.

**willosparky** · 15-11-2009

rcbo's do cost abit more tho but yh that is another option. the mk split load board will come with the main switch and the 2 rcd's.

also make sure the earth to the rod is 16mm

**Lenny** · 15-11-2009

My guess is he'll have sorted it by now as the thread is 6 months old.

**willosparky** · 15-11-2009

Originally Posted by lennytheloon

My guess is he'll have sorted it by now as the thread is 6 months old.

true but was replying just incase also may help someone else with same question

**electricAl** · 15-11-2009

Fair enough but accuracy is always appreciated by those looking for help.

The board you are referring to would actually be classed as a `High Integrity` board - not split load which offer some ways Rccb protected and some unprotected - as in 16th Edition.