Motor control circuits are typically fed from motor phase wiring by tapping phase circuits with control power transformer. This control voltage is typically 110-120VAC with neutral side of the control transformer grounded. With a grounded control power source there exists a possibility of inadvertent energization of motor if control circuit is incorrectly wired. This aspect is discussed in this article.

Consider a motor control circuit shown in figure 1. Motor is energized when the main contactor coil ‘M’ is energized which in turn close motor circuit contactor M. Control power transformer for this circuit is 120VAC phase-neutral with neutral [X2] grounded. Now, consider there is an accidental ground fault of one of wires in the clouded area. Ground fault will cause flow of current through the main contactor coil ‘M’ through the path shown in figure 2.

Flow of current through coil ‘M’ will energize the coil thereby closing the ‘M’ contacts on the motor circuit even though START switch is in off position. This will also bypass start switch thereby latching the start circuit ON until motor is manually stopped. This can be a very dangerous condition and is a significant safety issue. This type of accidental motor energization for the circuit shown in figure 1 can happen for the following conditions when ground fault occurs in the area clouded:

• Permanent ground fault in the control wiring
• Intermittent arcing ground fault
• Accidental ground fault during testing or troubleshooting

Note that even for intermittent ground fault, the circuit in figure 1 will ‘latch in’ and the motor will keep running until manually stopped.

Read: Autotransformer Starter

NEC Article 430.74

North American National Electric Code [NEC] article 430.74 has a section devoted to this potential safety concern. Article 430.74 states:

“Where one side of the motor control circuit is grounded, the motor control circuit shall be arranged so that an accidental ground in the control circuit remote from the motor controller will (1) not start the motor and (2) not bypass manually operated shutdown devices or automatic safety shutdown devices.”

Read: Motor Differential Protection

What is the correct method for motor control circuit wiring in compliance with NEC 430.74?

Circuit shown in figure 3 is the correct method for wiring motor control circuit that will be in compliance with NEC 430.74. Note that START switch is NOT on the grounded neutral side of control power source while relay coil M is in the grounded neutral side of control power source.

• An accidental ground fault anywhere between START switch to control power transformer fuse can only result in opening of the fuse due to short circuit. Ground fault in this area will NOT result in energization of motor relay M.
• An accidental ground fault anywhere from START switch to relay M will not result in any current as there is no voltage in this portion of circuit. Once the start switch is closed under this condition, there will be a phase to ground fault and control power fuse will open. There won’t be any accidental energization of motor relay M.

Read: Motor Current Calculator, Motor Starting Current Calculator