An often asked question is “Can You Fit A Metal Consumer Unit to a TT Earthing Arrangement System?“
Not so long ago, it was not allowed to have a metallic enclosure on a TT system.
The 3rd Amendment of the 17th edition of the wiring regulations, released in 2015, brought a requirement that all domestic switchgear was to be made from a non-combustible material.
This wasn’t a problem for Terra-Neutral (TN) systems as the external loop impedance is sufficiently low to ensure that if any live conductor comes into contact with the “Class 1” metallic enclosure casing, the fault would be cleared.
A low impedance allows sufficient current to flow to blow the upfront “supply” fuse in the service head, thus disconnecting the supply.
However, a TT earthing arrangement will have a much higher earth loop impedance. This will likely not be enough to disconnect the main fuse should a live wire touch the metallic casing of a consumer unit.
This could leave the outside casing of the consumer unit dangerously live, not even disconnecting when touched.
Not only that but anything connected to the metal consumer unit body would also become live, which includes anything connected to the MET, so bonded pipework, other class 1 equipment, etc….
Fitting Metal Consumer Unit to TT Earthing Arrangement System
Obviously this is a bit of conundrum.
So how should you go about safely fitting a metal consumer unit to a TT earthing arrangement supply?
You need to ensure that all possible chance of the meter tails coming into contact with the metallic enclosure of the consumer unit is reduced.
It used to be commonplace to install an “up-front” 100mA time delayed RCD across the whole installation to ensure that disconnection times were met.
This requirement is less pertinent now (on domestic installations at least) that RCBO consumer units are so common, providing individual 30mA RCD protection to each circuit.
In fact, the NICEIC technical helpline have confirmed that a 100mA up front time delayed RCD is no longer a requirement when the board is an RCBO equipped unit. In this instance only the busbar is not RCD protected.
There is a slightly different requirement that applies if the board is a dual RCD unit. In this case, where the tails between the RCDs are only singularly insulated, an “up-front” time delayed RCD must be fitted to ensure disconnection times are met.
However, dual RCD or RCBO board, some precautions must be taken on the incoming meter tails to ensure they stay secure and safe
Cables Within The Enclosure
Any cables within the metallic enclosure must be double insulated (more correctly termed ‘insulated & sheathed’, unless they are protected by an RCD.
The meter tails should be stripped back only the amount which is required for termination into the main switch. Excessive coloured core which has unsheathed basic insulation is a risk for damage and possible contact with the outer enclosure.
There is a particular issue with the tails within dual RCD consumer units between the main switch and the RCDs is normally only singular insulated cable. As discussed above, this would require that an up front RCD is used in order to be able to fit a metal consumer unit to a TT supply.
Cables exiting the protective devices does not need to be double insulated as on a modern distribution board this would be protected by an RCD, ensuring that the supply will disconnect if anything touches the metallic casing.
Meter Tail Glands
The best way to secure the meter tails where they enter the consumer unit is to use a proper meter tail gland.
These have grown in popularity since the requirement for non-combustible materials.
Fitting to a 32mm (or even 40mm) knockout in the consumer unit enclosure, these glands clamp tight over the tails ensuring that they cannot be snagged and pulled from the enclosure. They also have the added benefit of ensuring the IP rating of the side of the distribution board is maintained.
Although there are a couple of other ways of securing tails which wouldn’t necessarily be a “C2 – Improvement Recommended” issue (or even an issue at all in some cases!) when inspecting, these other methods usually do not combine the excellent strain relief with the high level of IP rating as using one of these glands.
These take any potential strain away from the terminations on the consumer unit main switch, helping to remove the possibility of the live conductor coming loose.
The glands also allow the main earthing conductor to enter through the same hole in the enclosure, to make sure that circulating eddy currents are not an issue.
Meter Tail Clamps
Meter Tail Clamps fit directly over the main switch, tightly gripping the meter tail cables.
These clamps are great for ensuring that there is no possible disruption to the main switch connections.
Although they can be used on their own, they are best when used in conjunction with a meter tail gland or the tails are secured tightly outside of the enclosure.
Secure Meter Tails Outside Of Enclosure
Like any cable, the meter tails should be correctly & adequately secured to ensure that no strain can damage to the terminations.
There are a number of ways this can be done, SWA cleats can come in handy, particularly “telecleats” which ensure a tight grip on the cable even when the sizes are not exact for a regular SWA cleat (as is often the case with meter tails)
One alternative is to use all-round band around the tails, clipping them securely to either the wall or cupboard to which the consumer unit is mounted. All-round band has the benefit of being fire resistant, ensuring compliance with the 18th edition requirements for fixing of cables.
Strain relief on the meter tails is the most important aspect of ensuring safety when fitting a metal consumer unit to a TT earthing arrangement system.
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:
- 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)
- Use a consumer unit which has internal meter tail clamps available (this is just about every manufacturer) – Use the clamp tightly.
- 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.
- Secure any loose meter tails outside of the enclosure such that the cable cannot be snagged in any way.
So Can You Fit A Metal Consumer Unit To A TT Earthing Arrangement System?
The basic answer to this is Yes, you can fit a metal consumer unit to a TT earthing arrangement system.
You just need to take a few basic precautions to ensure that the risk of the meter tails loosening from the main switch is lowered.
If you have anymore queries about fitting a metal consumer unit to a TT earthing arrangement system, then drop a comment below and we’ll get back to you.