By Mike Holt for EC&M magazine.
250.6 of the NEC specifies that objectionable current is not to flow on grounding and bonding paths but few understand what objectionable current is, why its dangerous and how to remove it.
What Causes Objectionable Current. Objectionable current on grounding and bonding paths occur when an improper neutral-to-case bond creates a parallel path for neutral current to return to the power supply via metal parts of the electrical system in violation of 250.142.
This happens when a neutral-to-case bond is made in a panelboard that is not part of service equipment or where a neutral-to-case bond is made at both the transformer and at the secondary panelboard. Improper neutral-to-case bonds also occur at generators, when the transfer switch does not open the grounded (neutral) conductor. Using the bonding path as a neutral conductor causes objectionable neutral current to flow on the bonding conductor. This happens when a 125V receptacle is needed at a switch location where a grounded (neutral) conductor is not available.
Danger from Objectionable Current. Improper wiring resulting in the flow of objectionable current on grounding and bonding paths can cause the temperature at loose fitting to rise to a level that can ignite combustible materials. In addition, arcing at loose fittings is particularly dangerous in areas that contain easily ignitable and explosive gases, vapors, or dust.
Objectionable current can result in dangerous voltage on metal parts of electrical equipment and death from ventricular fibrillation exits any time the voltage is above 30V. Graphic (to be published in the magazine)
Nuisance tripping of a ground-fault protection device can occur if neutral current returns on the equipment grounding conductor, instead of the neutral conductor. A circuit breaker with ground-fault protection (480Y/277V, 3-phase system over 1,000A) uses the residual current method to detect a ground fault [230.95]. On a 3-phase, 4-wire system, the trip unit will sum the currents in the three phase conductors and in the neutral. When no ground fault is present, the summation of currents flowing on A+B+C+N will equal zero. Any current flow not equal to zero is considered a ground fault.
Depending on the impedance of parallel neutral paths, the ground fault protective relay may see current flow above its pickup point and cause the protective device to open the circuit, when there is no actual fault.
If multiple improper neutral-to-case bonds exist and a ground fault occurs, the ground-fault protection relay might not operate because some of the ground-fault current returns on the neutral conductor, bypassing the ground fault protective device.
Another issue is low frequency electromagnetic interference. Current flowing on metal parts of electrical equipment and conductive building parts causes elevated electromagnetic fields in the building or structure. These low frequency electromagnetic fields can negatively impact sensitive electronic devices, particularly video monitors and sensitive medical equipment. Visit www.mikeholt.com/Powerquality/Powerquality.htm for more information.
What Can Be Done? As we can see objectionable current not only is dangerous but it can cause power quality issues. All that needs to be done to solve this problem is to ground and bond electrical installations in accordance with the requirements contained in the NEC.
Mike Holt's Comment: If you have any comments or feedback, please let me know, Mike@MikeHolt.com
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