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What causes lead cable overheating?

What's the likely cause of this cable failure in a 900 hp 208 volt chiller motor? The leads are multiple 250 kcmil cables. When the unit tripped off without any warning, we found unbalanced circuit resistances and in looking at the connections discovered one cable burned open inside the terminal box (which is quite large). But the connection hadn't overheated. The burning and the break were several inches away, inside the insulation, which wasn't destroyed.

Consider severe flexing of cables during motor starting. Unless leads are adequately supported, such movement can eventually break some of the strands, inside the insulation. That breakage increases the current-and heating-in the remaining strands, while contributing to easier flexing. Eventually the overloaded conductor opens up entirely. When several parallel cables form each phase lead, failure of one can cause the others to carry more of the load, so that the failure proliferates. Clamping or lashing the leads down should eliminate the problem..

Frame ground only for medium motors?

The larger, higher-voltage motors in industrial and utility plants always seem to have special ground pads on the frame, with a big cable connecting the frame to a building column or an underground grid. I've been told that such a connection is required only for motors over 600 volts. Is there an IEEE or other standard about which motors are supposed to be grounded that way?

This is primarily a National Electrical Code issue. In section 250.110, motor frames are required to be grounded under a variety of conditions, regardless of motor size or voltage. In NEMA MG 1, a frame grounding connection is required inside the motor conduit box for the smaller ("medium.") machines. The grounding conductor attached there is usually inside the motor conduit. This isn 't common with larger medium voltage machines, for several reasons. But some kind of frame grounding is a universal practice for safety reasons.

Edited by the EA staff

Copyright Barks Publications Jan 2005
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