Is this OK? 100m run of SWA

[scoul00]

Hi,

So i’ve recently put a 100m length of SWA 2-core 2.5 to the other end of a field with a 16a commando socket at the bottom. It’s for a shepard hut to go down the bottom to be plugged in and out.

The breaker it goes to inside the house where I’ve wired it in is a B16, onto the RCD that is already there (as there was 1 space left). The breakers inside the shepard hut are a B6 for lights and B10 for sockets. Only a microwave and kettle will be plugged in and occasionally a phone/laptop charger.

I ran tests yesterday, but i’ve only just passed my 2491-52 course (test and inspect). Insulation res was <500, earth loop between live and neutral is 1.79, with a PFC of 137A. Max in BS7671 is 2.7, so i thought that’s ok. However when i ran a test for the earth loop between live neutral and earth, it is 26.2ohm with a pfc of 9A. Am I right thinking this should be the same and it is way too high? Or is it safe to put power to because it’s just to do with the distance of cable in one run? Cheers

 

[mattg4321]

Hope you calculated the volt drop. Sounds like the cable is significantly undersized to me.

 

[telectrix]

if you've run 2 core SWA, using the armour as CPC, that's the problem.

 

[DPG]

I'm guessing for insulation resistance you meant >500 ?

 

[scoul00]

if you've run 2 core SWA, using the armour as CPC, that's the problem.

So the SWA isn’t enough protection on its own? If I ran another length of just earth cable down to the socket as the main CPC would that work?

 

[Julie.]

If you have passed your course, you should know how to calculate the expected resistances, a 2 core 2.5mm2 cable should have a core resistance circ 7.41 ohm per km, so 0.741 ohm for 100m, and armour resistance of 8.8 ohm per km, so you should expect r1+r2 of around 1.62 ohm, r1+ rn around 1.48 ohm.

Anything drastically away from this and there is something wrong!

Also 16a x 1.48 ohm = 23.68volts

This is over 10% volt drop in itself!

 

[scoul00]

Hope you calculated the volt drop. Sounds like the cable is significantly undersized to me.

I did and I tested the voltage down there. Read 244-245v. Measured supply and that is 251v. Just under the legal limit of 253v.

 

[westward10]

What is your earthing system, could explain your Zs.
Your cable is way undersized for vd unless the load is restricted to 6A, that is allowing 5% drop and not 3% for the lighting.

 

[James]

Unfortunately, I agree with the above comments.
Looks like you have under specified the cable.
Also, when you calculate it out before replacing it, I would be looking for a 20A breaker to feed it.

If you don’t, first lunch time, kettle goes on and microwave lunch at same time, breaker trips and you have to walk 100m in the snow to reset it!
Also, experience tells me if there is a socket in there then someone will add a 4 way extension lead and a fan heater as soon as the weather turns cold.
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I did and I tested the voltage down there. Read 244-245v. Measured supply and that is 251v. Just under the legal limit of 253v.

Did you measure the voltage down there with the kettle on or with no load on the cable?

 

[westward10]

Unfortunately, I agree with the above comments.
Looks like you have under specified the cable.
Also, when you calculate it out before replacing it, I would be looking for a 20A breaker to feed it.

If you don’t, first lunch time, kettle goes on and microwave lunch at same time, breaker trips and you have to walk 100m in the snow to reset it!
Also, experience tells me if there is a socket in there then someone will add a 4 way extension lead and a fan heater as soon as the weather turns cold.

Even for 16A you are looking at 10.0, 2-core over 100m at 5%.
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So the SWA isn’t enough protection on its own? If I ran another length of just earth cable down to the socket as the main CPC would that work?

Your line/neutral loop is 1.79 and your earth fault loop is 26.2, what does this indicate TT earthing system maybe?

 

[scoul00]

Even for 16A you are looking at 10.0, 2-core over 100m at 5%.
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Your line/neutral loop is 1.79 and your earth fault loop is 26.2, what does this indicate TT earthing system maybe?

Yeah the earthing arrangement is TT. Sorry fairly new to electrics I finished my apprenticeship a year ago. So is the high reading on my live neutral and earth because of the electrode? They don’t cover anything well in the new exams at all. And no the volt test was done with no load on the cable, the shepard hut hasn’t been plugged in yet.

 

[MJPD29]

Yeah the earthing arrangement is TT. Sorry fairly new to electrics I finished my apprenticeship a year ago. So is the high reading on my live neutral and earth because of the electrode? They don’t cover anything well in the new exams at all. And no the volt test was done with no load on the cable, the shepard hut hasn’t been plugged in yet.

Westward is right. At that distance you are looking at 10.0mm for 2 core. Im afraid you should have done your calculations previous rather than after completing the job.

 

[westward10]

Your earth fault loop impedance test is between line and earth (neutral connection is for the meter to function). This is the fault path and is often high due to an electrode being used, common on agricultural sites.
Volt drop must be measured with a load applied and in your case if the circuit was subjected to the full 16A the drop will be approximately 13%. For what you have installed you are restricted to 6A with that cable.

 

[DefyG]

Agreed undersized, just done a quick calc and came up with 16sq for 3% VD, and 10sq if 5%!

 

[davesparks]

Yeah the earthing arrangement is TT. Sorry fairly new to electrics I finished my apprenticeship a year ago. So is the high reading on my live neutral and earth because of the electrode? They don’t cover anything well in the new exams at all. And no the volt test was done with no load on the cable, the shepard hut hasn’t been plugged in yet.

What is the Ra of the TT system, or Ze if you have checked it that way?

Not that it matters too much at the moment because the new circuit is non-compliant due to the undersized cable.

 

[scoul00]

What is the Ra of the TT system, or Ze if you have checked it that way?

Not that it matters too much at the moment because the new circuit is non-compliant due to the undersized cable.

Just checked the Ze of the electrode. It’s 152ohm. Will close this thread for the mean time as I realise the wrong cable was selected. Fortunately it’s at my parents house so it doesn’t have to be exactly ‘to the regs’. I’m used to an industrial setting with 3-phase so it’s a different ball game. I’ll update when I sort it out and recalculate max demand etc. Thanks all for your help

 

[westward10]

It is no different it must comply to Regulations.

 

[davesparks]

Just checked the Ze of the electrode. It’s 152ohm. Will close this thread for the mean time as I realise the wrong cable was selected. Fortunately it’s at my parents house so it doesn’t have to be exactly ‘to the regs’. I’m used to an industrial setting with 3-phase so it’s a different ball game. I’ll update when I sort it out and recalculate max demand etc. Thanks all for your help

It does have to comply with the regulations, and personally I would consider this to be even more important when it is my own family who could be put at risk from non-compliances.

Cable calculations are exactly the same ball game whether it's industrial or agricultural, the only real difference is the additional rules for agricultural as its a special location.
I assume its agricultural as its has a shepherds hut and that implies a farm.

 

[suffolkspark]

Have you got an rcd of some description upfront on the cable for fault protection as its TT?

 

[Vortigern]

Did you bury the swa, maybe you could just run a parallel cable with it???? Hmmm maybe not as well.