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

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Resistance grounding is being 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)?


Resistance grounding is being 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)? 

Downsides to using an NGR:

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

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

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

4.      A failed (open) NGR can leave the system blind to ground faults

 

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

Systems where tripping is required use ground-fault relays and the protection is provided in devices such as circuit breakers or adjustable speed drives - assuming the required GF 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 properly size a grounding resistor by following our recommendations in our online document “Guideline on How to Size an NGR” linked at: https://www.benderinc.com/fileadmin/content/Products/d/e/How_to%20_size_an_NGRM.pdf

 

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 cases, lower-voltage LED lighting is used, supplied by a low-voltage single-phase distribution panel. In other cases, customers have opted to add isolation transformers to serve existing line-to-neutral loads. 

 

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

The resistor is a mechanical device and is exposed to some extremes.  It is often mounted outdoors and is therefore exposed to the elements in addition to severe thermal stress under fault conditions. In many designs and requests for quote the NGR is found in the scope of the transformer section. Since protection is typically in the switchgear section – in many cases this means the NGR is out of sight and out of mind.  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 be a result of 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 that were 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.


This article and any opinions it may contain are for informational and educational purposes only.  Bender has made every reasonable effort to ensure the accuracy and reliability of the information provided on this website.   However, the information is presented “as is” without warranty of any kind.  Bender does not accept any responsibility or liability for the accuracy, content, completeness, legality, or reliability of the information contained and/or opinions presented in articles or posts on this website.  No warranties, promises, and/or representations, expressed or implied, are given as to the nature, standard, accuracy or otherwise of the information provided in this website, nor to the applicability, suitability, or otherwise of the information to your particular circumstances.

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Downside to resistance grounding power systems