Length of tails between meter and consumer unit

[Gigsy]

Hi,

I am currently studying for my 18th edition regs update exam and came across a video by
SparkyNinja on you tube.

I think SparkyNinja is very good and produces high quality content.

However I disagree with his interpretation of regulation 434.2.1 (i)

In his video he says you need a fused isolator between the DNO's meter and a consumer unit if the cable is longer than 3 meters. I have heard this before but I do not think this is what the regulation is saying.

I think the regulation is saying you can put a fused isolator between the meter and consumer unit if you reduce the tails from 25mm2 to 16mm2 within 3 meters of the 16mm2 tails.

I am not sure if I am reading this right and would like to get the opinion of someone more educated than I.

Please find below a link to the video in question. The part I refer to is at time 47 minutes.

Am I right in saying you could use 25mm2 SWA for a run longer than 3 meters between the meter and consumer unit, without the need to install a fused isolator?

 

[davesparks]

Am I right in saying you could use 25mm2 SWA for a run longer than 3 meters between the meter and consumer unit, without the need to install a fused isolator?

No because DNO's do not permit the tails from the cutout to the customers first protective device to exceed 3m, technically I believe this is supposed to include the tails from cutout to meter.

 

[Ian1981]

Hi,

I am currently studying for my 18th edition regs update exam and came across a video by
SparkyNinja on you tube.

I think SparkyNinja is very good and produces high quality content.

However I disagree with his interpretation of regulation 434.2.1 (i)

In his video he says you need a fused isolator between the DNO's meter and a consumer unit if the cable is longer than 3 meters. I have heard this before but I do not think this is what the regulation is saying.

I think the regulation is saying you can put a fused isolator between the meter and consumer unit if you reduce the tails from 25mm2 to 16mm2 within 3 meters of the 16mm2 tails.

I am not sure if I am reading this right and would like to get the opinion of someone more educated than I.

Please find below a link to the video in question. The part I refer to is at time 47 minutes.

Am I right in saying you could use 25mm2 SWA for a run longer than 3 meters between the meter and consumer unit, without the need to install a fused isolator?

Overload and fault protection can be omitted at the origin of the installation as long as the supplier agrees that their ocpd affords protection from the origin to the consumer unit where further overload and fault protection devices exist.
Generally the suppliers will give a maximum of between 2-5 meters where they will allow their ocpd to protect the tails at the origin, any greater then they will not afford protection of the consumers tails and you will have to provide your own overload / fault protection

 

[Gigsy]

so are you saying that in a situation where the consumer unit is more than 3 metres
from the meter, you need another 100 amp fuse, immediately after the DNO's fuse? I don't see the point, what would that achieve?

 

[Ian1981]

It doesn’t necessarily need to be 100 amp, if an average 2-4 bedroom domestic installation then a 60/63 amp fuse will be more than adequate.
If The DNO will not allow their fuse to provide protection to the consumers tails then you need to provide such protection

 

[Gigsy]

It doesn’t necessarily need to be 100 amp, if an average 2-4 bedroom domestic installation then a 60/63 amp fuse will be more than adequate.
If The DNO will not allow their fuse to provide protection to the consumers tails then you need to provide such protection

What you are saying is a DNO policy. Does
regulation 434.2.1 (i)
sate this?
The way I read regulation 434.2.1(i) It is saying you may put a fuse in the tails within 3 metres from the meter if you reduce the cable size?

 

[Andy78]

See 434.3 indent (iv) and also 433.3.1 (iii)

 

[buzzlightyear]

been to a new build dno cut out side I recon about 8mters to the cu.

 

[Gigsy]

See 434.3 indent (iv) and also 433.3.1 (iii)

Just to verify we are on the same page.

My interpretation of regulation 434.2 is that a device providing protection against fault current shall be installed at the point where a reduction in the CSA applies.

Then regulation 434.2.1 goes on to state that a device for fault current may be installed under the following conditions
In the part of the conductor between the point of reduction of CSA and the position of the protective device
(i) not exceeded 3 m in length

So this regulation only applies when the 25mm tails from the DNO's fuse to the meter is reduced to 16mm2 from the meter to the consumer unit.

Is that correct?

So regulation 434.3 does not apply if you do not reduce the CSA of the cable and regulation 434.2.1 (i) does not state there is a 3m maximum length of meter tails?

 

[Andy78]

Just to verify we are on the same page.

My interpretation of regulation 434.2 is that a device providing protection against fault current shall be installed at the point where a reduction in the CSA applies.

Except where 434.3 applies, which is why I referenced it

434.3(iv) states that DNO rules permission must apply where their fuse is being used as fault protection. DNO rules will generally stipulate a length no more than 3m

 

[Gigsy]

Except where 434.3 applies, which is why I referenced it

434.3(iv) states that DNO rules permission must apply where their fuse is being used as fault protection. DNO rules will generally stipulate a length no more than 3m

But am I correct in saying this omission is relating to regulation 434.2

434.2 - A device providing protection against fault current shall be installed at the point where a reduction in CSA applies.

So regulation 434.3 is saying the omission of devices for protection against fault current need not be provided for (i.e. an omission required by regulation 434.2)
(iv) agreement with the DNO

So this is saying that you can reduce the size of the CSA and omit a protective device if the DNO agrees?

 

[buzzlightyear]

So this is saying that you can reduce the size of the CSA and omit a protective device if the DNO agrees

Isolation of meter tails at the origin

Regulation 537.1.3 states that, where the distributor provides a means of disconnection at the origin of the installation, it may be used, subject to the agreement of the distributor, as the means of isolation for the part of the installation between the origin and the main linked switch or circuit-breaker required by Regulation 537.1.4. This isolator can take the form of an integral switch within the meter (Fig. 1).

The distributor will usually require that a means of isolation is installed if the meter tails exceed a certain length. Distributors often stipulate different maximum lengths for meter tails but a maximum length of 3 metres is common.

A fused switch is recommended over 3 m by the Distribution Connection and Use of System Agreement (DCUSA).

The Note to Regulation 537.1.3 draws our attention to the fact that a cut-out fuse cannot be used for isolation by persons who are not authorized to do so by the distributor.

Regardless of whether or not it is required by BS 7671, there can be little doubt that the provision of an isolator at the origin of an installation is good practice. Where a contractor has called out the distributor to withdraw the cut-out fuse, a convenient opportunity is presented for an isolator to be fitted at the origin of the installation. This could be carried out either by the distributor or by the contractor. Thereafter, it would no longer be necessary to call out the distributor to withdraw the cut-out fuse.

 

[Gigsy]

Isolation of meter tails at the origin

Regulation 537.1.3 states that, where the distributor provides a means of disconnection at the origin of the installation, it may be used, subject to the agreement of the distributor, as the means of isolation for the part of the installation between the origin and the main linked switch or circuit-breaker required by Regulation 537.1.4. This isolator can take the form of an integral switch within the meter (Fig. 1).

The distributor will usually require that a means of isolation is installed if the meter tails exceed a certain length. Distributors often stipulate different maximum lengths for meter tails but a maximum length of 3 metres is common.

A fused switch is recommended over 3 m by the Distribution Connection and Use of System Agreement (DCUSA).

The Note to Regulation 537.1.3 draws our attention to the fact that a cut-out fuse cannot be used for isolation by persons who are not authorized to do so by the distributor.

Regardless of whether or not it is required by BS 7671, there can be little doubt that the provision of an isolator at the origin of an installation is good practice. Where a contractor has called out the distributor to withdraw the cut-out fuse, a convenient opportunity is presented for an isolator to be fitted at the origin of the installation. This could be carried out either by the distributor or by the contractor. Thereafter, it would no longer be necessary to call out the distributor to withdraw the cut-out fuse.

I was discussing regulation 434.2. I was asking if a device providing protection against fault current was needed (i.e. a fuse) if the tails were longer than 3 meters.

You are discussing a different regulation, concerning isolation.

 

[Keith Roszell]

Is there a law on meter tails or is there just a recommend length ?