Earth loop

[sparky147]

just finnished installing 5 x rcd sockets using 4mm cable run 60 meters using a 32amp type c mcb and getting a reading of 1.06 what would be the possible cause of the high reading

 

[malcolmsanford]

think he's trying to build up his number of posts with anything that comes to mind. have you seen all those threads by jhoncina. bloody rubbish.

That's my civil partner I have you know .....................really offended now

 

[telectrix]

That's my civil partner I have you know .....................really offended now

didn't think you knew anyone who was civil.

 

[Deleted member 26818]

Depends what cable he is using.
If he's using T&E, then I would expect an R1+R2 value of about 1.0026Ω, if using singles then about 0.5532.
Perhaps he should ring it.

 

[pushrod]

If it is a Zs of 1.06Ω i would do a Zs immediately after the mcb - sometimes they can have a surprisingly high impedance - any way it would allow a quick check of the R1+R2 and at least quickly point the finger in the direction of the problem.

 

[Chr!s]

Whats your Ze?

Whats the installation method?

Regards Chris

 

[Chr!s]

If the concern is about meeting your disconnection you need to consider the cables at there operating temp, so you need consider your installation method.

If your cable are to ref method A or B then your R1 + R2 = 0.57 at 70 C

 

[sparky147]

zs = 1.06 - leads 0.10 = 0.96 but just retested and the reading was lower at 0.91 - leads 0.10 =0.81 and the 4mm cables are singles run in plastic conduit, tomorrow going to retest disconecting the rcd sockets, so the 4mm cable at 60 meters is ok?

 

[Deleted member 26818]

I would suggest that you obtain a copy of GN3.
There is a chart near the back which gives maximum permissable Zs when measured at ambient temperature for various protective devices.
For a type C 32A MCB, the maximum is 0.58Ω and for a type B 1.16Ω.

 

[sparky147]

thought the max is 0.72 for a type c 32amp breaker

 

[malcolmsanford]

At 1.06 your Ze would be approx 0.5 is that what your getting for Ze or approx.

Now that you have told us it is Zs so your doing the LIVE test, and i'm pretty sure you will find that your getting this high reading due to the RCD sockets. As said do an R1 +R2 tomorrow with the sockets off and the cable joined by a chocy block, see what you get then.

 

[sparky147]

ok will do let you know the test results tomorrow also is the 4mm cable for 60 meters ok

 

[telectrix]

it's ok if all relevant conditions are met, .Ib, VD and Zs .

 

[Deleted member 26818]

thought the max is 0.72 for a type c 32amp breaker

The 0.72Ω is when the conductor is at a working temperature of 70°C.
When measuring, you would apply the 80% rule of thumb, to allow for the conductor being at the ambient temperature.
Adusted tables for an ambient temperature of 20°C can be found in the OSG Appendix 2 and in GN3 Appendix B.
There are also correction factors to be applied to the adjusted values for temperatures other than 20°C in the Appendicies.

 

[telectrix]

might be worth his while to contact the MCB manufacturer. reason i say this is that the BS7671 max. figures are a blanket value for all makes of MCB and the manufacturer may well quote a higher value.

 

[Engineer54]

As far as i know Tel, they all have to comply with the laid down ''common'' characteristics for each type of MCB. So even if there were some sort of difference between manufactures, i doubt if they'll tell you..lol!!!