Testing a TT System max ZS

[Traineeboy]

Just starting to look into TT systems a little more. When testing and recording results on a TT installation how does max ZS work ? 
 

i.e a 32a break type B has a max ZS or 1.37 but obviously when you take into account the ZE of a TT system that that will high. Is there a different table for max ZS for TT system.

I understand you have to have a RCD upfront but you still have to record Zs or each circuit. Not asking to be spoon fed but can people point me in the right direction please. 

 

[Sidney]

Use the maximum for RCDs as they're being used for fault protection, so 30mA would be 1667 ohms. Table 41.5.

Remember the MCB will protect against short circuit, line to neutral, and the RCD will protect against earth faults.

Although not common practice I tend to do a Zn test to confirm the MCB will disconnect within the required times.

 

[binky]

it is a bit confusing. Whilst 1667Ohms is acceptable with a 30mA RCD, I've always wworked on the basis that your dead test R1 + R2 figures should be as per expected for TN-S, and then add the rod figure. To me, the ccts are no different to any other earthing system, so you would expect the test results to match as a demonstration that your cct is good and you haven't got any loose connections or insulation faults anywhere.

 

[Traineeboy]

Ok thanks  guys. Am I right in thinking if you use an RCBO 30Ma board in a TT installation you don’t need an upfront RCD ? 
 

 

[Sidney]

The RCBOs need to be suitable for use on a TT system, check manufacturers instructions. If the tails are suitably supported, either clamped or glanded, then there's no requirement for an upfront RC, just a double pole switch.

 

[binky]

To expand on Sidney's point, RCBOs must be twin pole type that siwtch Neutral as well as live. It's a requirement that any residual current device switches the N as well as L, ie totally isolates the cct.

 

[Traineeboy]

Right got you. So then if you do have an up front RCD then the rcbo in the consumer unit can be single pole? Or I guess you could have to duel RCD as the RCD’s would be double pole. 

 

[Traineeboy]

To put this into wider context 

We have a job coming up for a TT system and want to get my head round it. This is what I think should be the setup, am I missing anything ? 

A Main switch fused to protect cable at the Origin of supply then 2 core SWA to an outbuilding. Note that the steel wire will be glanded at the supply end but not at the outbuilding end. The outbuilding is about 70 meters away from the supply so a 35mm2 is Being used to allow for volt drop. 

The SWA L and N conductors will go straight into a built in SPD in consumer unit in the outbuilding then to the main switch. The conductors will be via an SWA and clamped so no need for an upfront RCD. The board will be a duel RCD board and all circuits will be RCD protected.  Thoughts ? 

 

[Sidney]

The the source is a TN system and the outbuilding will be TT?

 

[Murdoch]

Op how many circuits? Load? Heating?

 

[binky]

To put this into wider context 

We have a job coming up for a TT system and want to get my head round it. This is what I think should be the setup, am I missing anything ? 

A Main switch fused to protect cable at the Origin of supply then 2 core SWA to an outbuilding. Note that the steel wire will be glanded at the supply end but not at the outbuilding end. The outbuilding is about 70 meters away from the supply so a 35mm2 is Being used to allow for volt drop. 

The SWA L and N conductors will go straight into a built in SPD in consumer unit in the outbuilding then to the main switch. The conductors will be via an SWA and clamped so no need for an upfront RCD. The board will be a duel RCD board and all circuits will be RCD protected.  Thoughts ? 

sounds good to me! 

 

[Traineeboy]

The the source is a TN system and the outbuilding will be TT?

Yes the source will be TN-C-S so can’t export Earthing. 

 

[Traineeboy]

Op how many circuits? Load? Heating?

The outbuilding board will serve 7 circuits. 

2 ring mains 

2 lighting circuits

1 small 2kw immersion heater 

1 circuit for 4kw heat mats 

1 air con unit circuit 

I’ve been told the Max Demand is going to be 50 Amps