The Downsides to Resistance Grounding Power Systems and How to Avoid Them

Resistance grounding is has become more widely adopted outside of the historical markets of mining, pulp and paper, and oil and gas. Several advantages of resistance grounding have been discussed at length in many white papers and articles. However, what are the downsides to using a neutral grounding resistor (NGR)?

1) Conventional ground-fault protection may have to be upgraded to increase sensitivity

Systems where tripping is required use ground-fault relays. The protection is provided in circuit breakers or adjustable speed drives - assuming the required ground-fault pickup sensitivity is available. If tripping is not required, more sensitive ground-fault detection or monitoring is still necessary to detect and alarm high resistance faults. Economical and compact single and multi-channel ground-fault devices are available for both new installations and retrofitting older designs.

2) Potential for transient overvoltages if the NGR is not adequately sized

You can learn how to size a grounding resistor properly by reading our online document, “How to Size an NGRM.”

3) Line-to-neutral loads cannot be served directly

On new installations, line-to-neutral loads are not used. In the case of upgrades or system retrofits, customers can replace line-to-neutral loads such as lighting with higher voltage line-to-line rated equipment. In some instances, lower-voltage LED lighting is used, supplied by a low-voltage single-phase distribution panel. Customers have opted to add isolation transformers to serve existing line-to-neutral loads in other cases.

4) A failed (open) NGR can leave the system blind to ground-faults can cause higher than anticipated touch potential and will have a higher risk of an arc flash

The resistor is a mechanical device and is exposed to some extreme climates. It is often mounted outdoors and is therefore exposed to the elements and severe thermal stress under fault conditions. In many designs and requests for quotes, the NGR has been found in the scope of the transformer section. Protection of this critical component has only recently been added to electrical codes such as the 2018 Canadian Electrical Code. It is overlooked as a failure point in many other electrical codes.

The causes of a failed resistor are varied but can result from vibration, lightning, under-rating, vermin, ice, mechanical deterioration due to age, etc. Other issues can cause an open circuit in the NGR path, such as theft of copper or broken bushings. No matter the cause, the result of an open resistor is an added risk to the electrical system. Unless the NGR is properly monitored, ground faults will go undetected, leading to a higher probability of phase-to-phase faults.

While less frequent, there is also the chance of a shorted NGR failure. There have been incidents caused by human error, and they can also be caused by worn insulation in conduit shorting the neutral to ground. In one case, a system with a shorted NGR later experienced a ground fault which led to an arc flash and lost time incident.

In the end, monitoring the NGR is critical to ensuring the system is protected. Bender’s LINETRAXX® NGRM700 is an advanced device that allows continuous NGR monitoring to provide relevant information of the system grounding. Contact Bender today to learn more about how we can provide you with optimal electrical safety devices.