GFP requirements for interconnected systems • Tatjana Dinic
January 18, 2017 - Ground fault protection (GFP) requirements in the CE Code are often misunderstood; FAQs range anywhere from the very basic to complicated engineered schemes.
GFP is needed for solidly grounded systems, where there is a danger from low-level arcing faults. Overcurrent protective devices (OCPDs) are designed to recognize faults in the overload and short circuit ranges—and initiate opening action accordingly—but, based on time-current curves, they do not detect low-level arcing faults.
Everyone is familiar with the GFCI provided at the receptacle or breaker to protect people from shock. GFCIs interrupt 6 mA of ground fault current or more, as defined in the CE Code. But not-so-familiar is GFP for the protection of equipment from damaging line-to-ground faults.
ESA has received several reports of GFP systems in projects with interconnected systems that, because they were improperly wired, did not provide the required protection.
Based on CE Code Rule 14-102(1), solidly grounded systems rated more than 1000A with voltages between 150 volts-to-ground and 750V phase-to-phase (or rated 2000A with voltages below 150) are required to be equipped with GFP.
(The rationale for these numbers was the high number of burn-downs reported on feeders and services operating in this voltage range.)
The CE Code states the GFP’s maximum setting shall be 1200A, while the maximum time delay shall be 1 second for ground fault currents equal to, or greater, than 3000A. This restriction minimizes the amount of damage done by an arcing fault, which is directly proportional to the time the arcing fault is allowed to burn.
There is more to meeting the requirements of Rule 14-102 than simply installing a breaker with trip configuration marked “LSIG”. Different ground fault sensing methods can be used to provide protection, but it is important to choose the appropriate one, especially where there are several sources, or interconnected systems with multiple grounding.
Often overlooked, Subrules 5, 6 and 7 of Rule 14-102 specify the requirements for sensors. The associated Appendix B Note further clarifies special precaution should be taken to ensure proper sensing by the GFP equipment when multiple systems are interconnected and grounded. An engineering study must be conducted to ensure fault currents do not take parallel paths to the supply system (and do not trip the breaker affected by the ground fault).
The diagram shows two buildings supplied from the main service and equipped with GFP. The ground fault sensing method (a.k.a. ground return method or ground strap method) is used in each. When there is a line-to-ground fault in Building B, multiple paths exist for the return of ground fault current to the source. Depending on its location, it is possible the GFP sensor will only see part of the ground fault current originating in its own system. For example, were a breaker in Building B set to maximum trip value of 1200A, it may see only a portion of the fault current (800A). As a result, the GFP in Building B does not operate when it should, creating a safety issue.
The selected GFP design and ground strap method is not the appropriate method for this installation. An alternative may be to ground the neutral only at the main service and provide GFP with the ground strap method here, or use the other methods of sensing fault current when GFP is provided at each building.
The same issues can be present where there are additional systems, such as backup generators connected to a building, with a transfer switch that is not switching the neutral.
It is recommended that ground fault protection of such systems be performance tested when first installed onsite, or an engineering study be conducted that can be made available to the authority having jurisdiction.
* This article also appears in the January 2017 edition of Electrical Business Magazine. Check out our ARCHIVE page for back issues.
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