Upgrade tails on CU replacement?

[andrew8902]

Hi. I have my NAPIT first assessment soon and I replaced a CU in a house which has 16mm2 consumer tails in place. I did not upgrade the tails to 25mm2, so my question is should I have upgraded them or am I allowed to leave the existing tails in place?
I do not know the rating of the DNO cut-out fuse
Yes the cut-out does have 100A stamped on the fuse carrier but that means nothing
Installation is 25 years old

Thanks
[automerge]1600593380[/automerge]

Hi. I have my NAPIT first assessment soon and I replaced a CU in a house which has 16mm2 consumer tails in place. I did not upgrade the tails to 25mm2, so my question is should I have upgraded them or am I allowed to leave the existing tails in place?
I do not know the rating of the DNO cut-out fuse
Yes the cut-out does have 100A stamped on the fuse carrier but that means nothing
Installation is 25 years old

Thanks

Also it was a Lewden RCBO board and I brought the cables through the knockout in the rear, am I required to use fire sealant in the rear section?

 

[telectrix]

16mm tails will be fine as long as they're not supplying a pot farm. mention to inspector that you've calculated the max load (allowing for diversity) at <80A.

no need for fire sealant unless you have broached a fire barrier.

 

[Dustydazzler]

I was generally under the impression that tails need to be updated to 25 on a 100a cut-out , but as tel says 16mm is fine in 99% of domestic situations and imo I would only ever use 16mm if the regs allowed it in all situations

as for fire foam , no you don’t unless it’s a ruddy great big hole in the wall

 

[ChrisElectrical88]

Be prepared to explain the reasons why they are 16mm tails and if you do what Tel says have your diversity calculations ready.
Hole in the back will be ok.
Might be asked if the RCBOs are Type AC or Type A and the reason for the selection.
Did you check all main bonding?

 

[Dustydazzler]

Be prepared to explain the reasons why they are 16mm tails and if you do what Tel says have your diversity calculations ready.
Hole in the back will be ok.
Might be asked if the RCBOs are Type AC or Type A and the reason for the selection.
Did you check all main bonding?

out of curiosity what are the fundamental differences between ac & a type rcbos

I believe only some manufacturers make both varieties

 

[buzzlightyear]

some times it's better to up grade to 25mm
Just in case they up grade more loading.
Car charger gear juccuci gear ect.. Ect.

 

[ChrisElectrical88]

Type A will do all of that of a type AC plus detect DC residual currents up to 6mA.
After watching a Sparky Ninga video earlier in the year I haven’t fitted a Type AC device since.

 

[pc1966]

The problem with the 'type AC' RCD is the current sensing transformer can be saturated quite easily by some DC component and it becomes blind to the AC part. I think some MFT used to use that trick for "no trip" Zs measurements.

Type A should have a bigger transformer and/or different magnetic core type so a "pulsed DC" waveform (i.e. AC & DC combined, say rectified mains) still produces enough of a measurement to fire the RCD trip mechanism. Cost should not be much more than type AC to make this.

I think that 'type B' RCD use a hall-effect sensor instead of a transformer's secondary winding so they can detect a smooth DC fault (as well as AC/pulsed DC), say leakage from a EV battery pack. Unfortunately the cost for parts here is significantly more than A/AC approach, but probably not as much as the catalogue price would suggest, that might be partly due to much lower demand, etc.

 

[ChrisElectrical88]

The problem with the 'type AC' RCD is the current sensing transformer can be saturated quite easily by some DC component and it becomes blind to the AC part. I think some MFT used to use that trick for "no trip" Zs measurements.

Type A should have a bigger transformer and/or different magnetic core type so a "pulsed DC" waveform (i.e. AC & DC combined, say rectified mains) still produces enough of a measurement to fire the RCD trip mechanism. Cost should not be much more than type AC to make this.

I think that 'type B' RCD use a hall-effect sensor instead of a transformer's secondary winding so they can detect a smooth DC fault (as well as AC/pulsed DC), say leakage from a EV battery pack. Unfortunately the cost for parts here is significantly more than A/AC approach, but probably not as much as the catalogue price would suggest, that might be partly due to much lower demand, etc.

Think your pretty spot on with your summery there. Although the price of A Types is now pretty low. Fusebox for example there about 4 quid more for the type A RCBOs. Type Bs are still expensive, up to £200 in some manufacturers.

 

[pc1966]

I suspect type AC will go fairly soon, as I don't think they are permitted in many EU places now, so no point in making two versions for any new design.

I don't really expect type B to drop massively though as they will only ever be used for things like EV chargers or PV supplies, which at best is going to be something like 10% or less of all applications. Fundamentally they are going to be more complex as well.

 

[ChrisElectrical88]

I don’t believe ACs are used in Europe in many country’s now, they were phased out over the last years for Types As. Frustrates me still when I ask for Type As in wholesalers and they ask me why I fit them when everyone else just sticks ACs in.

 

[AJshep]

I suspect type AC will go fairly soon, as I don't think they are permitted in many EU places now, so no point in making two versions for any new design.

I think your right! I did see a snippet of A2 (which is due to go to DPC soon) which Sparkninja put on his instagram, I expect you saw it too!
The wording was something along the lines of,"Type AC rcds can only be used to serve fixed equipment, where it is known the load contains no DC components(see note4)

Examples in note 4 being simple filament lighting and basic electric heating.

 

[pc1966]

I don’t believe ACs are used in Europe in many country’s now, they were phased out over the last years for Types As. Frustrates me still when I ask for Type As in wholesalers and they ask me why I fit them when everyone else just sticks ACs in.

Indeed.

But while AC are a couple of quid cheaper they will still be chosen by folks who don't know (or care) about the difference.

Though as said before, I suspect the real risk is small, but growing as more things have DC derived directly from mains used (e.g. washing machine motors).

 

[Mike Johnson]

I never understood the use of the AC type in France, when the A type was only a couple of € more, but the AC type is still available, I just wonder if its old stock being run down or they are still being manufactured.

 

[ChrisElectrical88]

Think we accidentally hijacked OPs post with this one ?.

 

[Mike Johnson]

All of my posts get hijacked why not this one as well.

 

[Vortigern]

I usually upgrade them, for one reason, I don't know what the main fuse is. And the cost is so little it ensures it is future proof as well. I also think belt and braces approach making sure everything can take whats thrown at it and a bit more besides.

 

[pc1966]

Wrangling the 19-strand 25mm seems easier than 7-strand 16mm, so less strain inside the CU as well. But in most cases no real need to change as free-air 16mm is easily able to cope.

E.g. Table 4D1A has 25mm vertical one-spaced method F (column 12) at 130A, and though it does not list 16mm for same conditions, applying the sort of ratio seen for method A (column 2) gives 99A

 

[nicebutdim]

Frustrates me still when I ask for Type As in wholesalers and they ask me why I fit them when everyone else just sticks ACs in.

You could politely suggest they stick to their job of selling stuff and leave specification to those qualified and well enough informed to decide what should be fitted.

 

[Dartlec]

Hi. I have my NAPIT first assessment soon and I replaced a CU in a house which has 16mm2 consumer tails in place. I did not upgrade the tails to 25mm2, so my question is should I have upgraded them or am I allowed to leave the existing tails in place?
I do not know the rating of the DNO cut-out fuse
Yes the cut-out does have 100A stamped on the fuse carrier but that means nothing
Installation is 25 years old

Another think to consider - Did you upgrade the main earth conductor from the MET?
It was/is common for 16mm tails to have a 10mm earth, but for PME I believe the regs state it must be 16mm.

If it's TN-S then maybe have the calculation ready if you are asked - or say that you've done it, with a confident look!
[automerge]1600721255[/automerge]

I don’t believe ACs are used in Europe in many country’s now, they were phased out over the last years for Types As. Frustrates me still when I ask for Type As in wholesalers and they ask me why I fit them when everyone else just sticks ACs in.

Wonder how long it will be before all the Screwfix £70 Dual RCD boards will be Type A?

 

[Mike Johnson]

Wonder how long it will be before all the Screwfix £70 Dual RCD boards will be Type A?

Not until they have sold all the in stock boards with the Type AC's in them.

 

[Ian1981]

Another think to consider - Did you upgrade the main earth conductor from the MET?
It was/is common for 16mm tails to have a 10mm earth, but for PME I believe the regs state it must be 16mm.

If it's TN-S then maybe have the calculation ready if you are asked - or say that you've done it, with a confident look!
[automerge]1600721255[/automerge]

Wonder how long it will be before all the Screwfix £70 Dual RCD boards will be Type A?

The regulations do not state that you must have a 16 mm earthing conductor, it can be sized by calculation, regardless of the line conductor but if pme conditions apply , needs to be at least 10 mm( domestically speaking)

 

[Dustydazzler]

The regulations do not state that you must have a 16 mm earthing conductor, it can be sized by calculation, regardless of the line conductor but if pme conditions apply , needs to be at least 10 mm( domestically speaking)

I always thought it was 16mm to avoid the need for mechanical protection

 

[Ian1981]

I always thought it was 16mm to avoid the need for mechanical protection

No, it’s sized upon table 54.7 or by the adiabatic equation.
If buried etc then other considerations for CSA apply.
New installations like new build houses, the DNO stipulate 25 mm tails and a 16mm earthing conductor.

 

[HappyHippyDad]

Hi. I have my NAPIT first assessment soon and I replaced a CU in a house which has 16mm2 consumer tails in place. I did not upgrade the tails to 25mm2, so my question is should I have upgraded them or am I allowed to leave the existing tails in place?
I do not know the rating of the DNO cut-out fuse
Yes the cut-out does have 100A stamped on the fuse carrier but that means nothing
Installation is 25 years old

Thanks
[automerge]1600593380[/automerge]

Also it was a Lewden RCBO board and I brought the cables through the knockout in the rear, am I required to use fire sealant in the rear section?

Hi Andrew,

Your assessor may or may not have a moan about the 16mm tails, each assessor seems to be different. Don't worry though, absolute worst case scenario is that you will have to upgrade to 25mm and that is unlikely.

I assess if 16mm tails need upgrading by looking at max demand and seeing if that is in excess of the CCC of 16mm cable. Also I check to see if it meets regulation 434.5.2, i.e protection of tails against fault current.

You could even show the above to the assessor as it is written in Best Practise Guide number 1 (Replacing a consumer unit) issue 4, section 5.1 note 4 as follows:

"16 mm² meter tails with a 100 A cut-out fuse could be adequate if the maximum demand of the installation (taking into account diversity and load profile) does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met."

You'll have to brush up on K² S² > I²t just in case he asks you to prove it meets the regulation though

Good luck

 

[Dartlec]

The regulations do not state that you must have a 16 mm earthing conductor, it can be sized by calculation, regardless of the line conductor but if pme conditions apply , needs to be at least 10 mm( domestically speaking)

Agreed that it can be calculated. I thought there used to be a specific PME reg, but apparently not currently - that is only when applying to bonding conductors.

Table 54.7 says that for a cross sectional area of Line conductor S up to and including 16, the protective conductor must also be S (if same material), so if not calculating and using that table 10mm earth would only be allowed on 10mm tails, and 16mm required on 16mm tails (then 16mm until S > 35)

 

[Ian1981]

Agreed that it can be calculated. I thought there used to be a specific PME reg, but apparently not currently - that is only when applying to bonding conductors.

Table 54.7 says that for a cross sectional area of Line conductor S up to and including 16, the protective conductor must also be S (if same material), so if not calculating and using that table 10mm earth would only be allowed on 10mm tails, and 16mm required on 16mm tails (then 16mm until S > 35)

Yes by design however if it’s existing there’s very little chance that a 10 mm earthing conductor will not satisfy the adiabatic so does not require to be changed.

 

[andrew8902]

Thank

Hi Andrew,

Your assessor may or may not have a moan about the 16mm tails, each assessor seems to be different. Don't worry though, absolute worst case scenario is that you will have to upgrade to 25mm and that is unlikely.

I assess if 16mm tails need upgrading by looking at max demand and seeing if that is in excess of the CCC of 16mm cable. Also I check to see if it meets regulation 434.5.2, i.e protection of tails against fault current.

You could even show the above to the assessor as it is written in Best Practise Guide number 1 (Replacing a consumer unit) issue 4, section 5.1 note 4 as follows:

"16 mm² meter tails with a 100 A cut-out fuse could be adequate if the maximum demand of the installation (taking into account diversity and load profile) does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met."

You'll have to brush up on K² S² > I²t just in case he asks you to prove it meets the regulation though

Good luck

Hi Andrew,

Your assessor may or may not have a moan about the 16mm tails, each assessor seems to be different. Don't worry though, absolute worst case scenario is that you will have to upgrade to 25mm and that is unlikely.

I assess if 16mm tails need upgrading by looking at max demand and seeing if that is in excess of the CCC of 16mm cable. Also I check to see if it meets regulation 434.5.2, i.e protection of tails against fault current.

You could even show the above to the assessor as it is written in Best Practise Guide number 1 (Replacing a consumer unit) issue 4, section 5.1 note 4 as follows:

"16 mm² meter tails with a 100 A cut-out fuse could be adequate if the maximum demand of the installation (taking into account diversity and load profile) does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met."

You'll have to brush up on K² S² > I²t just in case he asks you to prove it meets the regulation though

Good luck

Thanks for your reply. WPD are coming out today to confirm the type/rating of the cut-out fuse, which he suggested on the phone that they never fit 100A.
The tails have been in situ since 1995 and there are no signs of thermal damage, also I have examined with my FLIR E4 camera for hot spots. Maximum demand is around 50A
Cheers