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Thoughts on 3M max tails protection with SPD & isolator in tails?
Question
DNO allowance for tails protection from their fuse is commonly 3M. - 3M is deemed AFAICT to extent through any in-line isolator, whether or not that isolator is property of DNO or is consumer fitted part. (The latter is not spelled out by DNO SSEPD who  just state the 3M tails figure without further qualification).
If said isolator begins to sprout additional complexity such as MCB and SPD as a shunt addition, my interpretation (Definitions, part 2) is that it still doesn’t yet qualify as a consumer unit, hence the 3M rule could be said to still run straight through it as far as the farthest CU.
- Any thoughts?

 
12 Replies
AJJewsbury
3144 Posts

If said isolator begins to sprout additional complexity such as MCB and SPD as a shunt addition, my interpretation (Definitions, part 2) is that it still doesn’t yet qualify as a consumer unit, hence the 3M rule could be said to still run straight through it as far as the farthest CU.

The rule will run as far as the conductors that are only protected from overcurrent by the DNO's fuse - so the furthest switchfuse or CU that's directly connected to the supply. Remote CUs fed via a switch-fuse or a way in a CU next to the intake wouldn't be a concern.

  - Andy.
That is as I'd always thought it.
I picked out the "Fuse Box" brand F1M2SPD as potentially a very good  value way to provide up-front SPD for a garage installation which now has 4 CU's (EV, PV, heat pump, main house). Henley Block, no isolator. My part is technically only replacing the main CU. Fitting one of these isolator/SPD in series in the tails (ie not shunted off the Henley) before the Henley block appears to cover all bases with the bonus the SPD operates on the whole installation. 
However, it also struck me that though the SPD device is rated by the manufacturers as not needing MCB, (ie OK right on suppliers fuse), the internal 6mm wiring to the shunt device is technically a "circuit" protected by the main fuse. Not sure how this fits with the DNO view. (TNCS install BTW).

 

AJJewsbury
3144 Posts

the internal 6mm wiring to the shunt device is technically a "circuit" protected by the main fuse. Not sure how this fits with the DNO view.

I might be tempted to consider any factory fitted internals as being covered by the product standard rather than BS 7671 (or DNO rules) - as long as we use (say) 25mm² to the device then it's not our concern what's inside. Similar to the situation with bus-bars on many CUs that are often somewhat less than 25mm² c.s.a. (especially where they connect to the incoming device).

   - Andy.
Alan Capon
516 Posts
The 3m distance (or in my employer’s case 2.5m) takes you to the customer’s first over current device, whether this is a fuse switch or the consumer unit. Note the word “tails”. This is important, as the entire distance before the over current device must be in single meter tails. If you need to use SWA or some other multi core cable, then you will need overcurrent protection immediately before it. The reasoning is, the distance is used in the DNOs design calculations for both the normal arrangement and back feed options. The DNO cannot calculate each sub-main in each property, so it is easier to set a standard, and say that is what it must be. 

Regards,

Alan. 
Chris Pearson
3262 Posts
Take a pair (or quartet) of tails and put in an isolator. What has changed?
Is the '3m tails' in SSEPD territory written in stone, or is it subject to interpretation? We have been asked to do an EICR on an installation, and the first thing I noticed is that the CU is at the opposite end of the garage from the intake. The tails are circa 6m in length and enclosed in 25x40 minitrunking.  I suspect the Ze and PFC are still sufficient to take out the 60A service cutout but it did look odd to me.

Edit: the service is 60A, there's a sticker to the effect on the main fuse carrier, and the fuse has not been 'unsealed'. The tails are 25mm2, both from head to meter, and from meter to CU, no REC isolator present. The meter is NOT the type with the load terminals accessible without removing a seal, so it must have been sealed with those tails in place
mapj1
4444 Posts
A length limit of 3m after a 100A fuse  is not a BS7671 requirement, it is a DNO house rule that sets the length of tails . Given that you can then have another fuse of the same rating, and then as much cable as you like, it does not stand scientific scrutiny So long as any damage to the tails would blow the company fuse promptly, as far as BS7671 goes it is satisfactory. Subject to the usual things like rating, suitable routes etc.
It may be worth a note that it is 'unusual', and the DNO may not like to connect to it if they did it now, but fails no BS7671 requirement.
Alan Capon
516 Posts
I agree the 3m is a DNO requirement. They guarantee that the cutout fuse will operate for a fault within 3m. It may well operate for longer tails, but have all made the decision that they don’t have the time to do the calculations for each individual property. The customer must have over current protection after the first 3m, and rate it for their design of the extended tails, even if it doesn’t grade with the cutout fuse. 

Regards,

Alan. 
Sparkingchip
4840 Posts
So we can use the DNO main fuse to protect up to three metres of meter tails.

But presumably that is from the actual fuse, so includes the tails into the suppliers meter as well as the tails out to the consumer unit.

We can also use it to protect the main switch along with the internal tails and busbars in the the consumer unit, in addition we can use the suppliers fuse as backup if the prospective fault current is too high for the MCBs with the PFC being up to 16.5 kA with a device that is rated at 6 kA.

Now in some installations the DNO fuse is being used as the protective device for surge protection devices and their connections, with the SPDs mounted in the consumer unit, a separate enclosure or within a main switch enclosure.

It doesn’t actually matter if the meter tails are three metres long or less than a metre, the implications and risks are the same.

Andy Betteridge 
AJJewsbury
3144 Posts

A length limit of 3m after a 100A fuse  is not a BS7671 requirement,

It does sort of 'align' with a BS 7671 regulation though - i.e. the 3m requirement of 434.2.1. I'm sure at least one DNO (I can't remember which off the top if my head) has it written in their technical documentation that they offer no guarantees at all about what their fuse will provide for the consumer's installation, and so suggest using the 434.2.4 approach which allows a short length of BS 7671 circuit not to have overcurrent protection against faults. What that implies fo ADS with metal CUs isn't entirely clear.
   - Andy.
I think we all agree, thanks for the little discussion.

For MHRestorations: The 3M is stated in a fairly unqualified way (and there can be no real qualification possible, since,as has been pointed out, it is not part of BS7671). The key point of my post was to avoid upsetting DNO and/or ending up with any sort of business culpability by overstretching their ruling (in whatever way that may be possible). See "New connections guide Part 2" 
https://www.ssen.co.uk/connections/usefuldocuments/
 
Alan Capon
516 Posts

Sparkingchip:
So we can use the DNO main fuse to protect up to three metres of meter tails.

But presumably that is from the actual fuse, so includes the tails into the suppliers meter as well as the tails out to the consumer unit. . . 

Not quite. The distance is measured from the outgoing (load) terminals of the meter, or the outgoing terminals of the isolator where this has been provided by the supplier. 

Regards,

Alan. 

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