Submain breaker/fusing

[mbga9pgf]

All,

i LIve WITH a TT earthed system, thus struggle to meet 50v required disconnection times due to Ze value (around 15 ohms).

if you split the incoming supply on tt, are you required to provide

1) RCD protection only
2) Overcurrent protection only (with RCD protection at the submain consumer unit)
3) RCD and overcurrent protection (60a BS88 Fuse).

The submain routes underfloor, is 16mm t+e and supplies an annexe after a 25 meter run of cable.

Thanks in advance.

 

[Spark123]

You need to provide both RCD protection and overcurrent/fault current protection.

If the T&E isn't concealed in a wall or partition less than 50mm along any of it's length you may use a time delay RCD at origin and normal 30mA RCDs in the submain board.

 

[mbga9pgf]

You need to provide both RCD protection and overcurrent/fault current protection.

If the T&E isn't concealed in a wall or partition less than 50mm along any of it's length you may use a time delay RCD at origin and normal 30mA RCDs in the submain board.

cheers. Just had some work done by a decent registered chap, reccommended, who hasnt applied overcurrent protection to the submain before the submeter and has used a type B, rather than type S RCD....

16mm is in trunking until it goes beneath the floors

 

[Spark123]

Are you talking about the meter tails? I take it you mean a Type B MCB as opposed to an RCD, a type B RCD is for use with DC.
Can you upload a photo? Am off out now but will take a look later

 

[mbga9pgf]

Are you talking about the meter tails?
Can you upload a photo? Am off out now but will take a look later

I had the CU moved 3 metres and had all the wiring removed from the wall cavity... I have 25mm SWA bringing the supply up the cavity now, where its split in henley blocks, 25mm tails to the new CU and 16mm tails to a sub meter supplying the annexe. There is no Overcurrent protection on the 16mm tails, only 80a switchfuse protection from the incomer.

Whilst its unlikely, I am concerned that if the girl switches on all items in the annexe supplied by the 16mm submain, it could exceed the 60a limit on the junction boxes used in the submain, without protection.

Should have just done the job my bloody self and saved 400 quid...

 

[Spark123]

The SWA should be RCD protected (with a time delay RCD)

 

[mfarrow]

There is no Overcurrent protection on the 16mm tails, only 80a switchfuse protection from the incomer.

So you do have overcurrent protection then? A 16mm cable will just about take 80A if clipped direct.

Or did you mean fault protection (i.e. RCD)?

Whilst its unlikely, I am concerned that if the girl switches on all items in the annexe supplied by the 16mm submain, it could exceed the 60a limit on the junction boxes used in the submain, without protection.

Does the 80A fuseswitch actually contain an 80A fuse? Have you checked?

 

[mbga9pgf]

The SWA should be RCD protected (with a time delay RCD)

Thanks, its protected by a BS88 80A fuse, why does it need RCD protection as well? Disconnection times?

Its currently protected by a switchfuse after the supply fuse, I would have thought that RCD on the 25mm SWA would have been overkill as the SWA is earthed too?

As for the submain, I am more concerned about overcurrent than RCD protection on it,as it already has RCD protection at the next consumer unit!

 

[mbga9pgf]

There is no Overcurrent protection on the 16mm tails, only 80a switchfuse protection from the incomer.

So you do have overcurrent protection then? A 16mm cable will just about take 80A if clipped direct.

OK, so its less of an issue, other than I have a junction box connecting the 16mm submain and its max rating is 60A.... other issue I pointed out was I wanted the system to be easily upgradable to 100A, which I cant do now without the later addition of a 60A switchfuse, which is a massive pain.

Where can you get 16mm insulated crimps from to join the cable?

 

[Spark123]

If a live part touches the armour (if it was damaged etc) then not enough current will flow to disconnect a BS88 80A fuse, you have already stated Ze to be 15ohms.
230/15 = 15.3A, enough to trip a 100mA (S) RCD in time but nowhere near the required 580A to blow a 80A BS88 fuse in time, the fuse will just hold and cause all connected metalwork to become live at mains potential (unless of course there is an incidental path via main protective bonding lowering the Zs).

 

[mbga9pgf]

If a live part touches the armour (if it was damaged etc) then not enough current will flow to disconnect a BS88 80A fuse, you have already stated Ze to be 15ohms.
230/15 = 15.3A, enough to trip a 100mA (S) RCD in time but nowhere near the required 580A to blow a 80A BS88 fuse in time, the fuse will just hold and cause all connected metalwork to become live at mains potential (unless of course there is an incidental path via main protective bonding lowering the Zs).

Sure. But the SWA is deeper than 50mm, so would have thought on top of the earthed mechanical protection, no RCD would be required? THe Switchfuse was placed to meet the DNO requirements for one due to the length of the meter tails.

 

[RF Lighting]

No it MUST be RCD protected. It doesn't matter if it's buried or not.

You require RCD protection on that cable as in the event of a fault, the cable, and every piece of metal work in your house will come live. This presents a great risk of electric shock, and also the risk of fire.

The regulations for RCD protection of buried cables are for personal protection, and not to acheive disconnection times which is why you require one protecting this cable.

 

[mbga9pgf]

No it MUST be RCD protected. It doesn't matter if it's buried or not.

You require RCD protection on that cable as in the event of a fault, the cable, and every piece of metal work in your house will come live. This presents a great risk of electric shock, and also the risk of fire.

The regulations for RCD protection of buried cables are for personal protection, and not to acheive disconnection times which is why you require one protecting this cable.

So why the hell has a registered NICEIC electrician wired it up without one then? I thought the whole point of these bloody schemes was to ensure the work was safe?

 

[RF Lighting]

Sound's like your man doesn't have much expertise in the regulations for TT installations.

Was your man an approved contractor or a domestic installer?

Can you post some photos of the installation to double check that the things that apper to have been missed have actually been missed? We could also point out anything else that may be incorrect for you.

 

[mbga9pgf]

Sound's like your man doesn't have much expertise in the regulations for TT installations.

Was your man an approved contractor or a domestic installer?

Can you post some photos of the installation to double check that the things that apper to have been missed have actually been missed? We could also point out anything else that may be incorrect for you.

Will do cheers. Domestic installer with 5 years apprenticeship and 5 years in the trade. Will get some photos on my next visit, sounds like the easiest solution may be to get the DNO to get me connected up to local TNCS supply when they come round next; will solve most of the problems. Only other solution would be to swap the Switchfuse in the meter box with the RCD on the submain, replacing the existing 60a 30ma RCD with a 100ma DP S type between the meter and the 25mm SWA.

Thanks for the help so far.