High Impedance Grounded Neutral Safety Observations
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electrician2.com
High Impedance grounded systems as covered in Section 250.36 of the 2008 NEC are common in industrial installations where continuity of service is a priority. A phase to ground fault in these systems does not take the systems down and allows the maintenance personnel time to find the anomaly and correct it. A ground fault alarm may be energized for some time before the fault is cleared. This causes a rather unusual safety hazard while the ground fault is in effect. Since the ground fault places the grounding conductors at the same potential as the phase voltage the voltage between the other phases to ground remains at the phase to phase voltage until the fault is cleared. For older systems this is an added liability to insulations that may have been fine at the phase to ground fault potential but are now subject to a greater potential. It is something like a long endurance low potential high pot test for these conductors. I have seen a 480 volt system in a ground fault status for months at a time. In harsh out of door climates where snow grows to 24 feet in depth it is was not uncommon to wait until spring to clear ground faults. Unfortunately, this very situation existed in a classified area near large crude oil storage tanks that was a Class 1 Division 1 area until a second ground fault occurred on another phase causing a phase to phase bolted fault through the equipment grounding conductor material that existed at this site. Unfortunately, during the initial build of this facility 14 miles of cable tray called rigid cable support system in the 1975 NEC was installed using cables that had no equipment grounding conductors in them. This error was made with not only the 480 volt cables but in the 5 KV and 13.8 Kv cables as well. To compensate for the error the steel metal of the cable tray and conduits were bonded to form a compensatory equipment grounding system which in the case cited here had high enough impedance to prevent the 400 overcurrent device from tripping and caused arcing, sparking and smoke that set off quite a commotion amongst safety personnel. The conclusion to this little story which is true because I was there, is to clear ground faults ASAP especially when 800,000 barrels of highly volatile crude oil is flowing through the facility.
The second safety problem can be seen in Figure 77 below. In this diagram only one high impedance resistor is used and it is at the service. A neutral connection has been made from the grounding resistor XO side to the neutral point at the standby generator. The problem is that when the generator is off line and the service is online there is a potential of phase to neutral voltage between the de-energized generator windings and neutral point to the grounding conductors. A mechanic that may think the generator is off line may well find himself severely shocked if he works on the generator without having the interconnecting neutral disconnected. It might be a good practice to install a switch on the neutral for this purpose so the generator can be serviced without having to disconnect a hot neutral from the generator neutral point. Also signs should be used to warn workers about the possible hot neutral and windings.
electrician2.com
High Impedance grounded systems as covered in Section 250.36 of the 2008 NEC are common in industrial installations where continuity of service is a priority. A phase to ground fault in these systems does not take the systems down and allows the maintenance personnel time to find the anomaly and correct it. A ground fault alarm may be energized for some time before the fault is cleared. This causes a rather unusual safety hazard while the ground fault is in effect. Since the ground fault places the grounding conductors at the same potential as the phase voltage the voltage between the other phases to ground remains at the phase to phase voltage until the fault is cleared. For older systems this is an added liability to insulations that may have been fine at the phase to ground fault potential but are now subject to a greater potential. It is something like a long endurance low potential high pot test for these conductors. I have seen a 480 volt system in a ground fault status for months at a time. In harsh out of door climates where snow grows to 24 feet in depth it is was not uncommon to wait until spring to clear ground faults. Unfortunately, this very situation existed in a classified area near large crude oil storage tanks that was a Class 1 Division 1 area until a second ground fault occurred on another phase causing a phase to phase bolted fault through the equipment grounding conductor material that existed at this site. Unfortunately, during the initial build of this facility 14 miles of cable tray called rigid cable support system in the 1975 NEC was installed using cables that had no equipment grounding conductors in them. This error was made with not only the 480 volt cables but in the 5 KV and 13.8 Kv cables as well. To compensate for the error the steel metal of the cable tray and conduits were bonded to form a compensatory equipment grounding system which in the case cited here had high enough impedance to prevent the 400 overcurrent device from tripping and caused arcing, sparking and smoke that set off quite a commotion amongst safety personnel. The conclusion to this little story which is true because I was there, is to clear ground faults ASAP especially when 800,000 barrels of highly volatile crude oil is flowing through the facility.
The second safety problem can be seen in Figure 77 below. In this diagram only one high impedance resistor is used and it is at the service. A neutral connection has been made from the grounding resistor XO side to the neutral point at the standby generator. The problem is that when the generator is off line and the service is online there is a potential of phase to neutral voltage between the de-energized generator windings and neutral point to the grounding conductors. A mechanic that may think the generator is off line may well find himself severely shocked if he works on the generator without having the interconnecting neutral disconnected. It might be a good practice to install a switch on the neutral for this purpose so the generator can be serviced without having to disconnect a hot neutral from the generator neutral point. Also signs should be used to warn workers about the possible hot neutral and windings.
posted by Gerald Newton at 3:08 PM
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