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Under what specific conditions is a grounding resistor (NGR) useful, and where its not useful?

Hi Engr,

Thank you for your question regarding Neutral Ground Resistors, it is our pleasure to help.

The selection of a Neutral Ground Resistor is one of the main choices in deciding what type of electrical system your facility will have.  There are four (4) types of electrical systems:

  1. Ungrounded Delta Systems
  2. Solidly Grounded Neutral Systems (this is the most common by far)
  3. Low Resistance Grounded Neutral Systems
  4. High Resistance Grounded Neutral Systems

The first system is a Delta power system, and the last three are Wye power systems.  The choice of which system to use is extremely important and a fundamental decision that will impact your entire electrical system.  Generally, only power companies deal with #1 (Ungrounded Delta Systems) for their power distribution from substation to substation.  For most businesses and homes today use item #2 (Solidly Grounded Neutral Systems), as this system is the best for ensuring that any ground faults will maximize current flow enabling Over Current Protection Devices (OCPD) such as circuit breakers and fuses to function properly.  Until recently, items # 3 and #4 were considered to be dangerous options as circuit protection technology was not sufficient to ensure that OCPD would protect personnel and equipment.  In other words, resistive grounded neutral systems (#3 and #4) require advanced technology in their OCPD’s that simply wasn’t available until recently.

Today, resistive grounded systems (#3 and #4) are becoming more and more popular, as the technology to provide proper circuit protection becomes more and more available.  The advantages/disadvantages between the four (4) systems above would require a chart and graphs in order to keep track of them all.  But needless to say, the decision impacts a variety of engineering factors that will affect your entire electrical system, including:  transient over-voltages, voltage stress, arc-fault, safety to personnel, equipment reliability, ability to detect ground faults, equipment costs, multiple voltages on the same system, frequency faults, ground fault current flows, training requirements, downtime, and compliance with local electrical codes.

The selection of when and where to use a neutral ground resistor is a very important decision (possibly the single biggest decision you can make), that actually has very little to do with grounding per se.  It certainly impacts grounding, but the reasons for making the selection often have nothing to do with earthing or grounding.  As such, E&S Grounding Solutions is probably not the right company to help you with this important process.  We recommend you contact Electro Industry at 714-776-5599 (California time) right away.  They will be happy to help you.

Best regards,

The Engineering Team at E&S Grounding Solutions


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