TT advise

[NCSPARKY]

Hi everyone ,

I have a new build Rewire coming up. Currently the meter is in a green cabinet at the front of the driveway and the new house will be around 30 metres away. I have done all the calculations for the cable which is fine. The earthing system is currently a TT system with a spike (will renew) behind the green cabinet. I will be installing an RCBO consumer unit in the house. Currently there is a “temporary” supply which has a switch fuse in place, my question is would I need to protect the new SWA cable from the cabinet to the new house with an RCD rather than a switch fuse as a TT system, I was thinking about a fuse box housing with a 100ma Type S RCD so no to not interfere with upstream RCBOs? Any advice would be great first real time coming across this exact scenario! Also would there need to be earth rods near the house or will the earthing at front of driveway be ok. Thanks J

 

[PS Electrics]

Hi everyone ,

I have a new build Rewire coming up. Currently the meter is in a green cabinet at the front of the driveway and the new house will be around 30 metres away. I have done all the calculations for the cable which is fine. The earthing system is currently a TT system with a spike (will renew) behind the green cabinet. I will be installing an RCBO consumer unit in the house. Currently there is a “temporary” supply which has a switch fuse in place, my question is would I need to protect the new SWA cable from the cabinet to the new house with an RCD rather than a switch fuse as a TT system, I was thinking about a fuse box housing with a 100ma Type S RCD so no to not interfere with upstream RCBOs? Any advice would be great first real time coming across this exact scenario! Also would there need to be earth rods near the house or will the earthing at front of driveway be ok. Thanks J

Came across this a few months ago.

My research indicated there is no need for up front RCD protection on the tails (With an RCBO board) on a TT system so long as they are provided with enough protection to the point of termination in the Consumer Unit.

So Tails Clamp is Mandatory.

Problem you will have is ordinary SWA doesn't have double insulated conductors.

So I think the easiest solution is an 100ma Type S RCD like you have stated.

 

[mhar]

The SWA doesn't need rcd protection (assuming the armouring is correctly earthed.
The 100mA time delayed rcd isn't required by our regs but is advisable from a belt and braces point of view but would terminate the swa into this enclosure at the consumer unit end - otherwise the occupants would have quite a walk if it did trip

 

[Yellowhammer]

Is there much point in installing the RCD at the CU end though. You could just terminate the armoured into an enclosure and come out with the tails. The biggest function of the upstream RCD here to my mind is protection of the SWA. Putting it upstream you're back to arguing that it's not needed as in a 'normal' TT installation.

FWIW last TT install I did, I did fit a 100mA S in place of a 2 pole iso. So I'm not necessarily against it, more devil's advocate.. However in this instance it seems you could either argue to ommit it completely, or put it inconveniently 30m walk from the front door..

 

[DefyG]

As there is an existing rod cannot you not determine the existing Ze (or better it with a new rod) and do the calcs to see if anything other than a RCD will provide protection required in time required (2s)?

 

[newfutile]

I can't see you ever getting a low enough Ra or ZE to meet disconnection times on a fuse or mcb, have you asked if they will pme the supply?
A line to earth fault in the swa may never clear in any time.

 

[PS Electrics]

The SWA doesn't need rcd protection (assuming the armouring is correctly earthed.

Should we NOT be earthing the armoring in this case if doing away with the RCD protection? On a TT we won't meet the disconnection time requirement for an earth fault and so if the SWA is earthed and such a fault occurred we have just made the whole installations earthing live!

The 100mA time delayed rcd isn't required by our regs but is advisable from a belt and braces point of view but would terminate the swa into this enclosure at the consumer unit end - otherwise the occupants would have quite a walk if it did trip

Problem here is ensuring that no part of the submain right up to termination at the CU can come into contact with the Installation Earthing.

Normally the only part to be concerned with on a TT system is the metal CU. However we won't have double insulated conductors in the SWA so we need a joint to double insulated tails, that for me swings it to an S Type 100mA RCD 30m from the property.

If there was a SWA with double insulated conductors we could terminate straight into the CU then I would be more inclined in that scenario to omit the up front RCD.

 

[PS Electrics]

As there is an existing rod cannot you not determine the existing Ze (or better it with a new rod) and do the calcs to see if anything other than a RCD will provide protection required in time required (2s)?

I have never heard of a TT rod ever meeting ZE requirements for a Fuse or MCB and even if it did you would have to worry that was under extremely favourably conditions that won't be there all year round.

 

[Pretty Mouth]

my question is would I need to protect the new SWA cable from the cabinet to the new house with an RCD rather than a switch fuse as a TT system, I was thinking about a fuse box housing with a 100ma Type S RCD so no to not interfere with upstream RCBOs?

You will need both RCD (live to earth fault) and fuse (line to neutral fault, overload) to protect the SWA between meter cabinet and house CU, and the armour of the SWA must be earthed.

 

[mainline]

You often hear “electricians” say that you don't need an upfront RCD or that it is not a requirement, which is true.

But that is only true if you can meet the requirements, In the case of a TT earthing system this is highly unlikely.

 

[mainline]

If there was a SWA with double insulated conductors we could terminate straight into the CU then I would be more inclined in that scenario to omit the up front RCD.

How would having double insulated cores on SWA protect against cable damage causing a L to E fault ? On a TT earthing system.

 

[mainline]

So Tails Clamp is Mandatory.

Where does it say it's Mandatory ?
A stuffing gland would meet the requirements.

522.8.5 Every cable or conductor shall be supported in such a way that it is not exposed to undue mechanical
strain and so that there is no appreciable mechanical strain on the terminations of the conductors, account being
taken of mechanical strain imposed by the supported weight of the cable or conductor itself.

 

[PS Electrics]

Where does it say it's Mandatory ?

My Opinion it is Mandatory without an upfront RCD.
I think the tails clamp gives that extra support to prevent any possibility of the tails coming loose which we absolutely want to prevent on a TT with no up front RCD protection.

A stuffing gland would meet the requirements.

§22.8.5 Every cable or conductor shall be supported in such a way that it is not exposed to undue mechanical
strain and so that there is no appreciable mechanical strain on the terminations of the conductors, account being
taken of mechanical strain imposed by the supported weight of the cable or conductor itself.

Tails Clamp + Stuffing Gland it was a technical article on TT Systems with RCBO or Split Board Consumer Units and the need or not for an upfront RCD. Mainly because of the switch to Metal Consumer Units in the 17th Edition Amendment 3.


The best way to ensure that there is the absolute lowest chance of the meter tails being disturbed is to take a 4 way approach:

1. Make sure to only strip the absolute minimum away from the outer sheathing of the meter tail. The entirety of the meter tail cable within the enclosure must be ‘double insulated’ (more correctly ‘insulated & sheathed)
2. Use a consumer unit which has internal meter tail clamps available (this is just about every manufacturer) – Use the clamp tightly.
3. Fit a round meter tail gland on the penetration through the consumer unit. Ensure it is clamped to the enclosure tightly and also ensure the cable is securely gripped by the gland.
4. Secure any loose meter tails outside of the enclosure such that the cable cannot be snagged in any way.

 

[PS Electrics]

How would having double insulated cores on SWA protect against cable damage causing a L to E fault ? On a TT earthing system.

It wouldn't the idea was not to have any joint to double insulated tails.

 

[mainline]

My Opinion it is Mandatory without an upfront RCD.

Nobody is saying it's not a good idea or that it's not good practice, but it's not mandatory.

Mainly because of the switch to Metal Consumer Units in the 17th Edition Amendment 3.

There wasn't a switch to strictly use a metal consumer unit, but one made of non-combustible material.
You could still use a plastic Cu, in fact, plastic ones are preferred in some locations.

If there was a SWA with double insulated conductors we could terminate straight into the CU then I would be more inclined in that scenario to omit the up front RCD.

Just because you may have double insulated Swa isn't a good enough reason to omit the upfront Rcd that's being used for fault protection.


PS Electrics said:
Should we NOT be earthing the armoring in this case if doing away with the RCD protection? On a TT we won't meet the disconnection time requirement for an earth fault and so if the SWA is earthed and such a fault occurred we have just made the whole installations earthing live!


The Swa armour Must be earthed, otherwise a penetration of the armour could cause fatal electrocution.