maximum zs

[Andy B]

What table do I use in the on site guide to find the maximum Zs for a TT sytems? My Ze is 3.61 so they all be over anyway?

 

[telectrix]

very true spin but you have to put in a 30ma RCd in for final circuits and it is the end of the final circuit you take te ZS from which is 1667 ohms

not necessarily. RCD not required on non-S/O circuits (excepting special locations) unless cables are <50mm deep in walls without earthed metallic conduit. what spinlondon means is that these non-RCD protected circuits have a max. Zs of 500ohms on a TT system.

 

[Deleted member 26818]

very true spin but you have to put in a 30ma RCd in for final circuits and it is the end of the final circuit you take te ZS from which is 1667 ohms

Not all final circuits require 30mA RCD protection, for instance: if the installation were a workshop, there may be no requirement for 30mA RCD protection other than on socket-outlets used to supply mobile equipment outdoors, and that could be provided by the use of an RCD socket-outlet. As such all the final circuits could then be covered by a 100mA RCD.

 

[JUD]

I'm going to throw a spanner in the works now with regards to max Zs on TT when using RCD.

Max disconnection time for TT is 0.2 secs as Tel said earlier. However BS EN 61008 RCDs are designed to operate at ≤ 0.3secs at 1 x IΔn (I know in practice they usually go a lot faster than this)

To guarantee they operate in less than 0.2secs you need a residual current of 2 x IΔn, so 60mA for 30mA RCD and 200mA for 100mA RCD (see Table 3A on page 243 BRB).

So for 30mA max Zs would be 50V ÷ 0.06A = 833Ω and for 100mA max Zs would be 50V ÷ 0.2A = 250Ω

 

[scotsparky]

Not all final circuits require 30mA RCD protection, for instance: if the installation were a workshop, there may be no requirement for 30mA RCD protection other than on socket-outlets used to supply mobile equipment outdoors, and that could be provided by the use of an RCD socket-outlet. As such all the final circuits could then be covered by a 100mA RCD.

remeber the thread is for TT system where rcd protection has to be fitted. TN-C-S systems allow for ommitions.

 

[scotsparky]

I'm going to throw a spanner in the works now with regards to max Zs on TT when using RCD.

Max disconnection time for TT is 0.2 secs as Tel said earlier. However BS EN 61008 RCDs are designed to operate at ≤ 0.3secs at 1 x IΔn (I know in practice they usually go a lot faster than this)

To guarantee they operate in less than 0.2secs you need a residual current of 2 x IΔn, so 60mA for 30mA RCD and 200mA for 100mA RCD (see Table 3A on page 243 BRB).

So for 30mA max Zs would be 50V ÷ 0.06A = 833Ω and for 100mA max Zs would be 50V ÷ 0.2A = 250Ω

thats one of the reasond that it is recomended that the Max earth rod resistance is no more than 200ohms

 

[Deleted member 26818]

remeber the thread is for TT system where rcd protection has to be fitted. TN-C-S systems allow for ommitions.

There is no requirement in BS7671 where TT systems must have RCD protection.
Regulation 411.5.2 One or more of the following types of protective device shall be used, the former being preferred:
(i) An RCD
(ii) An overcurrent protective device.
The requirements for additional protection by use of a 30mA RCD apply to all installations, as do the exceptions.

 

[Des 56]

(i) An RCD
(ii) An overcurrent protective device.

That is akin to saying a man with no teeth can munch on ice cream or hazelnuts

Both are permissable,but ice cream is preferable and possibly the only practical choice

 

[scotsparky]

heres an example if no RCD is provided and there are high ZS readings due to a Ze of 30 ohms due to the earth rod how are you going to satisfy disconnection times, prove in the event of fault theat any voltage on exposed conductive parts is under 50v?

 

[JUD]

There is no requirement in BS7671 wher TT systems must have RCD protection.
Regulation 411.5.2 One or more of the following types of protective device shall be used, the former being preferred:
(i) An RCD
(ii) An overcurrent protective device.
The requirements for additional protection by use of a 30mA RCD apply to all installations, as do the exceptions.

Did you read Note 1 under 411.5.2??

 

[Deleted member 26818]

Don't think I can be bothered any more.
Perhaps it would be easier scotsparky if you ran through the reasons why you consider 30mA RCD protection is required on all circuits in TT installations, and why you believe the requirements for 30mA RCD protection are differnt for TT installations than for TN installations.
Yes JUD I am aware of Note 1 of Regulation 411.5.2

 

[IQ Electrical]

Did you read Note 1 under 411.5.2??

The Ra value would have to be sufficiently low to allow the use of an overcurrent protective device, this can be achieved by using underground structural metalwork/earth plates etc. but in practice, it's almost impossible with the cheap, thin rods that I find are usually used on TT systems.

 

[johnboy6083]

The Ra value would have to be sufficiently low to allow the use of an overcurrent protective device, this can be achieved by using underground structural metalwork/earth plates etc. but in practice, it's almost impossible with the cheap, thin rods that I find are usually used on TT systems.

it would not only have to be lowe, but also stable. we all know that in this country water levels rise and fall, asnd if your insdtalling rods, just after rain, then you will have reasonablly low readings. do the same test after a heat wave, and they will be a lot higher.
the best way is depth. get your rods so deep that they are unaffected by rainfall. also install more than one, linked together.

Ive read about ufer earthing, and it sounds interesing to say the least. Im going to speak to our electrical engineer about it to see if it can be applied to some of our new sites that are having civils done at the same time.

 

[malcolmsanford]

Out here in the desert mate we use the Ufer system of earthing, or out here Grounding for all builds. I prefer to actually weld a lug onto the ree bar in the foundations but it seems that the above clamps are the clients spec.

 

[scotsparky]

Don't think I can be bothered any more.
Perhaps it would be easier scotsparky if you ran through the reasons why you consider 30mA RCD protection is required on all circuits in TT installations, and why you believe the requirements for 30mA RCD protection are differnt for TT installations than for TN installations.
Yes JUD I am aware of Note 1 of Regulation 411.5.2

1 Unable to garentee a stable earth suitable for disconnection times for other forms of overcurrent protection

2 BS 7671 4113.3 ,415.1.1 all final circuits

 

[Deleted member 26818]

(1) With TT systems, the requirement is to provide either overcurrent or RCD protection (411.5.2)
Where an RCD is used to provide earth fault protection, (411.5.3) requires that final circuits have a maximum disconnection time of 0.2 seconds, and distribution circuits have a maximum disconnection time of 1 second.
It further requires that RA x IΔn < 50V.
These requirements are considered met if Zs meet the requirement of Table 41.5.
Table 41.5 list the maximum Zs for RCDs ranging between 30mA and 500mA.
There is no requirement that the RCD has to be 30mA.
(2) 411.3.3 details the requirements for RCD protection of socket-outlets with a rated current not exceeding 20A that are for use by ordinary persons and are intended for general use, and for mobile equipment used outdoors. This Regulation, as I have already pointed out applies to all types of installation, and nowhere doese it indicate or even suggest that all circuits of a TT installation requires 30mA RCD protection.
415.1.1 details the required characteristics for an RCD which is used to provide additional protection. Again nowhere does it indicate or suggest that such additional protection is required for all circuits of a TT installation, or even for all final circuits of a TT installation.