Ground Fault Testing: Why It Matters
A ground fault occurs when an energized portion of a grounded electrical system becomes unintentionally connected with a path to “ground” or “earth” (through personnel, equipment enclosures, grounded conductors, conduits, fish-tape, etc.). When this happens, the system phase-to-ground voltage can cause harmful and excessive levels of current to flow through the unintentional path.
The magnitude of unwanted ground current varies depending on a variety of system conditions. Higher levels of current in the short-circuit or 5-10x overload range can be quickly detected and cleared by the overcurrent protective device (fuse, circuit breaker). Lower levels of arcing-type currents are not so easy to detect and can rapidly escalate into highly destructive phase-to-phase or multi-phase faults.
Prior to 1971, detection of low-level ground faults was neither required nor routinely employed in electrical systems in the United States. As electrical distribution systems and equipment proliferated across the country and higher voltage levels became more of the norm, instances of equipment-related fires and burn-downs increased.
The National Fire Protection Association (NFPA) recognized that even low levels of uninterrupted ground fault currents can cause extensive equipment damage and facility fire hazards. To address this, the NFPA’s National Electrical Code (NEC) began requiring ground fault detection and protection on electrical service entrance equipment rated 1000 amps or more operating at 480Y/277V.
In 1978 the NEC went one step further, requiring on-site testing of the ground fault systems. As stated in NEC Art. 230.95(c), “The ground-fault protection system shall be performance tested when first installed on-site. The test shall be conducted in accordance with instructions that shall be provided with the equipment. A written record of this test shall be made and shall be available to the authority having jurisdiction.”
The NEC is a regionally-adopted standard; in many places, the Authority Having Jurisdiction (AHJ) is the only equipment inspector and code enforcer available. In some cases, the local AHJs don’t enforce the NEC ground fault test requirement, and if they do, they may consider the “push-to-test” button on the ground fault control panel to be an acceptable means of satisfying the NEC requirement — it’s not. The “push-to-test” feature only verifies that a portion of the ground fault system is properly installed and functional; it does not verify the entire sensing and protection system.
Owners and electrical contractors may assume that new electrical equipment is fully tested by the equipment manufacturer before it ships. While true in a few cases, manufacturers are only required to test a small portion of their equipment to satisfy UL requirements; rarely is all equipment fully tested. In fact, proper and careful installation of the equipment protected by the ground-fault system is necessary. Since minor variances from the manufacturer’s intended arrangement can have serious detrimental impacts on ground-fault system performance, it’s crucial that the fully-installed system is tested by a qualified and knowledgeable consultant to ensure proper ground fault system installation.
What can go wrong in a new ground fault system?
Looking at three decades of HP&D’s testing reveals that approximately 10% of ground fault systems fail when tested properly, and more complex systems are more likely to have problems. Common issues include:
- Incorrect wiring of ground fault control power and/or current transformers (CPTs and CTs)
- Main, tie and/or feeder breaker neutral sensors (CTs) installed in the wrong location or with reverse polarity
- Ground or neutral current sensing CTs with incorrect ratios
- Main bonding jumper missing or connected in the wrong location
- Neutral and/or ground conductor landed on the wrong switchgear bus (neutral conductors on the ground bus and/or ground conductors on the neutral bus)
- Neutral conductors landed on an incorrect neutral sensor
- Control power turned off or unavailable
- Incorrectly sized (or missing) CPT fuses
- Ground fault relays shipped and set at minimum settings (standard) with potential for causing nuisance tripping
- Component failures (faulty or miswired breaker auxiliary contacts, shorted or open-circuited CTs)
What do we recommend?
Have your ground fault system field-tested whether the AHJ requires it or not. Hire a qualified independent testing consultant to perform a full review of your ground fault system installation and documentation, and to field test your system.
Your testing consultant should use a primary current injection test set to properly test your system – not just the “push-to-test” button. With your ground fault protection system powered-up and operational, your testing consultant connects the current injection test set to act as the main “current-source” for your switchgear, switchboard or main circuit breaker. Temporary jumpers are then connected to simulate normal “electrical loads” or unwanted “phase-to-ground faults.” The test engineer evaluates the response of the ground fault system to different levels of current in these various current-flow conditions.
Ground fault field testing requires a scheduled outage and takes more time than a simple “push-to-test” procedure, but it is the only way to effectively prove proper installation, sensing, and performance of your ground fault protection system. You may be surprised by what your testing consultant uncovers.