Motor earth

[Jako13!]

I’ve recently seen a motor wired directly from the local isolator to the motor terminal box in 3 core 1.5mm cable. The gland has been made off into a threaded hole in the motor terminal box and at the isolator end directly connected to the incoming swa from the MCC by means using a flylead. My question is is this a sufficient for earthing the motor. My understanding is you can use the armour as a cpc but wasn’t sure if any additional “fly leads” should be fitted at the motor end?

 

[James]

it may well be ok, have you tested the earth continuity?

there is little point in adding a fly lead if the gland is directly screwed into the device.

 

[Jako13!]

it may well be ok, have you tested the earth continuity?

there is little point in adding a fly lead if the gland is directly screwed into the device.

Yeah earth continuity from the motor terminal earth to the isolator is coming up as 0.01 ohms

 

[Jako13!]

Yeah earth continuity from the motor terminal earth to the isolator is coming up as 0.01 ohms. I’ve just seen sometimes flyleads coming of the gland to the main chassis of the motor what would the reason for this be?

Thank you

 

[James]

years ago, and it is still possible now but rarely done,

entire installations were done in rigid steel conduit.
the conduit would be the earth, all the cables inside would be phase or neutral.

extra earth cables were rarely added as the earth resistance through the conduits was normally better than you could achieve with an earth cable of equal or smaller csa to the live conductors.

the only time fly leads were added was if the fitting or device being powered needed an earth and it was not directly bolted to the conduit.

 

[James]

because they have always done it like this,

think back to basics, if you have a bolted connection to the motor terminal box, that is then bolted to the motor itself, you are likely to have a better connection and lower resistance than any fly lead could achieve.

also, if the terminal box is plastic then a fly lead would be required.

 

[Jako13!]

because they have always done it like this,

think back to basics, if you have a bolted connection to the motor terminal box, that is then bolted to the motor itself, you are likely to have a better connection and lower resistance than any fly lead could achieve.

also, if the terminal box is plastic then a fly lead would be required.

Thank you James