To Connect together or not

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kikay0115

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Taytay, Rizal, Philippines
I have a 75KW, 480 v, 3-phase motor. The cable supplied is a 185mm2 - 4C(R-Y-B-Bl) copper shielded cable. My problem is this, due to a very limited space at the motor cable box - do I have to connect the 4th (black)cable to the ground/earthing together with the copper shield or just leave the black cable floating? What is the safest/ideal way to do it? Thanks guys in advance:)
 
kikay0115 said:
I have a 75KW, 480 v, 3-phase motor. The cable supplied is a 185mm2 - 4C(R-Y-B-Bl) copper shielded cable. My problem is this, due to a very limited space at the motor cable box - do I have to connect the 4th (black)cable to the ground/earthing together with the copper shield or just leave the black cable floating? What is the safest/ideal way to do it? Thanks guys in advance:)
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Under the NEC, the EGC gets bonded without question. I assume from your description that is the black-insulated conductor (under NEC identified as such using approved means). If the motor is not powered by a VFD, connect drain (shield) also. If powered by VFD, terminate shield (or not) per manufacturer's instructions.
 
kikay0115 said:
Will it not pose a problem(e.g. -capacitance) if I cannot the EGC and the black cable together at both end?
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Apparently we have a misunderstanding. One of the conductors must be the EGC. Either the shield, if it qualifies as an EGC, or one of the other conductors... which by your description is the black. Whichever it is, that must be connected at both ends... and I'm leaning toward the black being the EGC.

If the shield qualifies as the EGC, then it gets connected at both ends, and you float the black conductor at both ends (JMO).


If the shield is not the EGC (which means the black is the EGC), then the shield should be grounded according to VFD manufacturer's instructions... but IMO at the very least one end (typically the supply end).
 
Anything capacitive coupling between the wires and the shield will not result in much current and the voltage difference at the open end will not prevent the shield from performing its function of reducing external electric field effects. Leaving the shield disconnected at one end may help reduce the ground current effects which could cause problems for communication, control or analog wiring between the two endpoints.
But for a pure power cable I would just follow the VFD instructions rather than try to reason it out.
:)

Tapatalk!
 
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