Where to measure Ze

[Harry Worth]

Sorry in advance for what may be a daft student question. Where should I measure my Ze? My mum has a TNCS system with the meter/incoming supply at one end of her bungalow. The CU is some distance away (I guess around 15m) and connected via T&E. Do I attempt to measure Ze at the incoming or at the CU. There is no obvious place at the meter to take a reading easily. On my course we always do it in the consumer unit but of course that's not real life.

 

[malcolmsanford]

Ze at the origin of the installation. As your "tails" are 15 meters away I assume you must have them off a switch fuse or some other protection device, so I would do my ZE there.

If the tails have no protection device and come straight out of the head/meter to the CU for 15metres, then I would be thinking of putting one in, as those tails are not protected.

 

[Big n Daft]

"The CU is some distance away (I guess around 15m) and connected via T&E."
As the lid peeled off the can and baby worm poked his head out into the daylight...........................................

 

[Knobhead]

Oh dear I can see this developing :furious3:

 

[Harry Worth]

Ze at the origin of the installation. As your "tails" are 15 meters away I assume you must have them off a switch fuse or some other protection device, so I would do my ZE there.

If the tails have no protection device and come straight out of the head/meter to the CU for 15metres, then I would be thinking of putting one in, as those tails are not protected.

The only protection device is the main 100A fuse on the other side of the meter. There is an isolation switch fitted however.

 

[malcolmsanford]

Well Harry, by the way loved your programme, then you can do your Ze from the isolation switch, but your going to have to replace that switch eventually with a switch fuse as the DNO will not allow their main fuse to give you fault protection on tails those lengths.

 

[mickys86]

Being a student myself. I got a question

I know the tails need an isolator if longer than 3 metres but would the cpc be sufficient for the C/U if it was connected in PVC/PVC? seeing that the cpc is considerably smaller than the conductors?

 

[mickys86]

Ps. Welcome Harry 

 

[Octopus]

Well Harry, by the way loved your programme, then you can do your Ze from the isolation switch, but your going to have to replace that switch eventually with a switch fuse as the DNO will not allow their main fuse to give you fault protection on tails those lengths.

Maybe, maybe not.Unless the tail lengthis "flagged" to the DNO they're highly unlikely to come and look at it! Having said that use T&E as tails isn't the best solution!

 

[malcolmsanford]

Maybe, maybe not.Unless the tail lengthis "flagged" to the DNO they're highly unlikely to come and look at it! Having said that use T&E as tails isn't the best solution!

So you would not be wanting to fit a protection device for those tails and rely on the main fuse for overload/fault protection.

 

[Harry Worth]

Maybe, maybe not.Unless the tail lengthis "flagged" to the DNO they're highly unlikely to come and look at it! Having said that use T&E as tails isn't the best solution!

Knew I shouldn't have promised my mum to sort this one out. Is it simply the case that the CU should really have been located close to the meter rather than the other end of the bungalow. I take it that this arrangement doesn't comply with regs. What would it be noted as on a PIR - i.e. requires urgent attention, requires improvement etc.

 

[Octopus]

So you would not be wanting to fit a protection device for those tails and rely on the main fuse for overload/fault protection.

Yes, I would recommend that the tails are replaced and protection for the tails is added too.

The point I was trying to make is that the DNO's don't like long tails but they don't exactly go out of their way to inspect for them, nor make such recommendations to the Public generally, which means muggins like you and me have to try and convice hard pressed home owners to part with money, etc etc.!!

 

[Octopus]

Knew I shouldn't have promised my mum to sort this one out. Is it simply the case that the CU should really have been located close to the meter rather than the other end of the bungalow. I take it that this arrangement doesn't comply with regs. What would it be noted as on a PIR - i.e. requires urgent attention, requires improvement etc.

If I was doing a PIR I would code the tails/lack of overload protection as a code 2 - others may have different opinions.

 

[malcolmsanford]

Harry have a look at reg 434.3 (iv) and it will tell you what you need to know.

The DNO will not supply fault protection to tails that exceed certain lengths, and that is normally 3-5 metres and the exact length is dependent on 1 the protection device in the head 2, Size of incomer cable 3. The DNO stipulation.

I would be totally amazed if they would would give you protection on 15metres of tails.

It's not hard really mate, just change the isolator to a switch fuse, an hours work, after all it is your mum's home

 

[Harry Worth]

Harry have a look at reg 434.3 (iv) and it will tell you what you need to know.

The DNO will not supply fault protection to tails that exceed certain lengths, and that is normally 3-5 metres and the exact length is dependent on 1 the protection device in the head 2, Size of incomer cable 3. The DNO stipulation.

I would be totally amazed if they would would give you protection on 15metres of tails.

It's not hard really mate, just change the isolator to a switch fuse, an hours work, after all it is your mum's home

Hi Malcolm - Thanks for that. I'm not registered as yet with a Part P scheme - I was going to move and replace the CU as a registration task. My mum's bungalow was a refurb from around six years ago. I am beginning to wonder whether the device I've identified as a simple isolator is in fact a protective device. There is no current rating on it so I've assumed it's an isolator. My mum has asked me to move/lower the CU so she has access to it (it's up at the ceiling in the back of an airing cupboard and totally inaccessible without steps even for me. This had me looking at the location of everything including of course the tails which I need to extend (I was going to replace completely until I found out the cost). I wasn't expecting this arrangement and it's also got me thinking about the capacity of the T&E cable. How do you identify csa of a cable.

 

[malcolmsanford]

A 10mm Twin and Earth cable will be roughly 17mm x 10mm and a 16mm T+E is 20mm X 10mm.

The CPC will vary to which size cable you have.

 

[Big n Daft]

I can see daddy worm now:willy_nilly:

 

[Geordie Spark]

Sorry in advance for what may be a daft student question. Where should I measure my Ze? My mum has a TNCS system with the meter/incoming supply at one end of her bungalow. The CU is some distance away (I guess around 15m) and connected via T&E. Do I attempt to measure Ze at the incoming or at the CU. There is no obvious place at the meter to take a reading easily. On my course we always do it in the consumer unit but of course that's not real life.

I'm just wondering what size this bit of T&E is ....... and how big is the fuse "protecting" it.

 

[mickys86]

i can see daddy worm now:willy_nilly:

lol 

 

[moorman84]

I would put a BS88 fuse instead of the isolator as it will give a higher maximum Zs. Also it the T&E running through thermal insulation as this greatly reduces the current rating of the cable. All of this should be taken in consideration when deciding the size of the fuse. For a TNCS system the minimum size of your earth cable should be 10mm.