This often comes up as an issue when dealing with un-grounded systems. This message was recently sent by me to my staff and I thought it might be of value to post it here as well.
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CEC rule 10-106(2) (and similar in the NEC) has special requirements for ground fault detection equipment on un-grounded AC systems. So, what do we need to remember in regards to this rule?
First… AC systems are either…
1) Solidly grounded, meaning that the system neutral or star point or the middle conductor of a 3 wire circuit is tied to the grounding system. It is also possible that a solidly grounded system could also be corner grounded as well, which means one phase and not the neutral is tied to the grounding system, though such systems are very rare (I have seen only one of them in my time in the trade).
2) Impedance grounded, meaning that the neutral or star point is tied to the grounding system through a low or high impedance resistor. Why would we do this, simply to reduce the level of fault current that would flow during a fault event.
Reducing the fault current can reduce damage to wiring and equipment during an event. Available fault current is never reduced to the point where the system over current devices cannot still operate as required.
3) Un-grounded, meaning there is no connection between the system and ground in any fashion. Why would we want to do this? Because an un-grounded system can help ensure the availability of a system in respect of “up time”. If we had a dead short between a phase conductor and ground on an un-grounded system, the system runs as per normal, that is one reason that un-grounded systems are very often used in waste water handling systems.
The danger with an un-grounded system is that, if one phase was in fact shorted to ground and we did not know that it was so shorted and if a second phase shorted out, the fault current would be much higher as the fault was now at phase to phase potential and not at phase to neutral potential.
So, the requirement in 10-106(2) for un-grounded system is that ground fault detection equipment is to be provided on the un-grounded system.
Attention must be paid in regards to what type of equipment is used for this purpose. Ground fault detection equipment is very different for use on an un-grounded system then the equipment used on a grounded system.
On a grounded system, basically we measure (via a CT) the current flow in all un-grounded system conductors. Thanks to the fact that the vector sum of the current flows sums to zero, we can monitor the flow in the bonding or grounding system. If the vector sum through the CT is not zero, then current is flowing in a path it is not supposed to flow in and we can then react to that fact.
On an un-grounded system the above CT based detection method is not possible as there is no current flow until a second phase conductor becomes grounded as well. So, ground fault detection equipment of un-grounded systems in fact monitors the integrity of the insulation system on all phase conductors.
This is an important distinction as ground fault detection equipment designed for grounded systems will not work on an un-grounded system and vice versa. Here is a link to a company that produces ground fault detection equipment meant for use on un-grounded systems.
Note: this is not an endorsement for the equipment from this company.
https://www.bender-ca.com/products/ground-fault-monitoring-ungrounded-overview
Ground Fault Indication on Un-grounded AC Systems
Re: Ground Fault Indication on Un-grounded AC Systems
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