# TT system

Discussion in 'Electricians' Talk' started by mick761, Mar 8, 2009.

1. ### mick761New Member

Being an urban house basher i don't often come across a TT system. Now we have the 17th how does that affect the RCD on a TT system. It used to be a 100mA rcd covering the whole dwelling. What with dual rcd board and RCDs with a 30mA rating, which RCDs do you fit? I don't see any point in fitting the 100mA RCD if I've got the two 30mA, is that right?

2. ### phiwaiNew Member

Yes your correct as far as if all your circuits are protected by 30ma rcd at the origin of the supply then no need for 100ma rcd as main switch.
You would still have installations that would require 100ma rcd as main switch but if this is a domestic property you are talking about then what you say would be fine.

It's all about your disconnection time.On 230volts with 100ma the max ohms you can have to get 0.4 sec disconnection is 500ohms ( note the reg also says it should be 200ohms or less).For a 30ma rcd ohms law says you could have 1667ohms and still get 0.4 disconnection but again you need a reading of 200 or less to meet regs.

3. ### 50Hz ShuffleNew Member

Not quite right.

TT disconnection time is 0.2secs.

Also the max.Zs for RCD disconnection is based on the maximum allowed so that the RCD will trip at 1xIn.

The RCD disconnection time is Ra x In = 50v (or less).

(In = I delta n)

4. ### phiwaiNew Member

TT disconnection time is 0.2secs if you do not have an overcurrent device and no protective equipotential bonding connected to all extraneous-conductive parts.

So you go with 0.2 on your TT systems as you've not put
any bonding in or used overcurrent devices then?

Also look at note 2

Where disconnection is provided by an rcd the disconnection times in table 41.1 relate to prospective residual currents significantly higher than the rated residual operating current of the rcd (not 1 times In)

5. ### 50Hz ShuffleNew Member

Sorry, but if you think Zs on TT systems is low enough to cause automatic disconnection by an overcurrent protective device, you are seriously over egging the average TT system.

Take a look at Appendix 3 and note some of the data.

My own TT system measures a best Zs of just over 8ohms..now exactly which circuits would achieve 0.4s disconnection via a fuse or mcb? I'd be lucky to achieve it on 6A Type B, let alone anything higher.

Any yes, I have all protective equipotential bonding in place

6. ### 50Hz ShuffleNew Member

Also, it would appear that you have not yet amended your copy of BS7671:2008 with the Corrigendum issued by the IET in JULY 2008.