LV Cables

[neps3]

Hi All

Could you please give me the minimum insulation resistance of 415V cables (power) and the standard which applies.

Thanks

 

[DanDel]

NETA spec is 200 megohms minimum for 600V and below at 1kV test voltage.

 

[eemotor]

If the cables are to be used for a motor, IEEE Std. 43 recommends that minimum insulation in megohms be greater than machine rating in kilovolts plus 1. So for 415V, you'd need 1.415 Megohm minimum insulation resistance.

 

[neps3]

Thank you Dandel and eemotor

 

[neps3]

eemotor

Whilest a industry practice is to accept kV+1 MegOhms as acceptable minimum the IEEE43 has changed. For equipment built after 1970 the IR minimum is 100MegOhm for one minute.

Could you please point me in the right direction as to which clause in IEEE43 states that for cables used for a motor should be kV+1 MEgohms.

Or by engineering judgement do we have to accept the minimum value for cables as that of the motors also. I agree with this judgement. We cannot have two values anyway. One for the motor and one for the cable.

The only issue is that cables for LV motors are either PVC or XLPE whilst some motors may be class F in worst ambient condition.

Could you please comment.
Thanks

 

[busbar]

Aside — IEEE Std 43-1974 Table of Contents. Standard 43 can be purchased in PDF form.

 

[neps3]

busbar

There is a new revised version of IEEE43 which is IEEE43-2000. I have read this copy and it does not talk about cables but motors.

Any comments on the cable insulation please.

Thanks

 

[busbar]

At shop.ieee.org, enter 43-2000 in the search field.

 

[neps3]

busbar

I requested comments on the IR not shopping details. Please read my last comment.

Thanks anyway.

 

[eemotor]

The reference to Std. 43 was taken from NEMA Standards Publication MG 1-1998 (Rev. 3, 2002). 12.62 (b) of NEMA MG 1-1998 states "Using 500 volts direct-current, take a 10 minute insulation resistance measurement. The insulation
resistance value shall be not less than the minimum recommended in IEEE Std 43. (Insulation
resistance in megohms = machine rated kilovolts plus 1.)" However, as you mentioned, this does not specifically indicate the minimum insulation requirements for power cables. This is more of a winding specification.

 

[neps3]

eemotor

Thanks and that was quite helpful.

Would you please give me the formula for calculating the IR in terms of insulation material constant, diameter etc. There are no values specified in any documents except for the works testing methods. It is not a question of installing the cable and forgeting until it fails.

As said my view is that the cable IR should not be less than that of the equipment to which it is connected. Can we accept a different lower value?

Any comments welcome please.

Thanks eemotor again

 

[CJFlatters]

Guys

As you state that the voltage is 415V, I assume you are therefore referring to a UK system, this would therefore come under the IEE Regulations (BS 7671) not the NEC, NEMA or IEEE43 (not that there is anything wrong with those standards - they just dont generally apply in the UK).

BS 7671 Reg 713-4, Table 71A Applies

For an LV (415V) system, the DC test voltage should be 500V and the minimum resistance should be 0.5 Megohm, in practice is is unlikely that the actual reading should be anywhere as near as low as this anyway unless you have an old cable.

Above 500V, the DC test voltage required is 1000V and the minimum acceptable reading would be 1 Megohm.

Hope this helps.




_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager