Electrical System Protection for Worker Safety
- Published: Monday, 09 January 2012 12:29
Do You Really Know if Your Electrical System is Safely Grounded?
By Jeff Glenney
Equipment such as conveyors, crushers and vibrating screens are susceptible to electrical ground faults due to rock dust, rain, cable abrasion and improper connections. The result is the potential for a dangerous shock hazard for workers. That’s why MSHA 30 CFR 56/57 dictates the use of a high-resistance grounding system that limits touch potential to a safe level.
The problem is that the system itself is a piece of equipment that is too often neglected. With a broken grounding resistor, ground-fault protection devices stop working, and equipment and workers are put at risk. There are no symptoms when the resistor fails; many quarry and aggregate operators may have an ungrounded system right now and not know. To explain how to solve this, a little background on electrical systems is needed.
A grounded system is just what the name implies: the central (neutral) point of the three-phase transformer feeding the system is connected to earth ground (Fig. 1). This is intuitively simple, and everyone understands it, but if an energized conductor faults to ground – through cable or equipment damage, or because of a dropped tool inside an electrical cabinet – there can be an Arc-Flash: an instantaneous release of huge amounts of energy in the form of light and heat, a dangerous blast wave, and flying bits of molten metal, as occurred at a surface granite mine in Oklahoma in October 2010, killing a contract apprentice electrician and seriously injuring two co-workers.
In an ungrounded system, nothing is connected to earth ground; the neutral point is electrically “floating” (Fig. 2). The advantage is that the circuit breakers will not open if there is a ground fault (equipment can continue to operate). The disadvantage is that a ground fault may shock workers who touch equipment or water in which a ground fault is traveling. Also, ungrounded electrical systems can be subject to high-voltage electrical transients that can damage equipment.
Resistance Grounded System
To get around these problems, the resistance grounded system (also called a high resistance grounded – HRG – system) was invented, and is required by many regulations, including 30 CFR 56/57, CSA M421-00, “Use of Electricity in Mines.”
In a resistance grounded system, the neutral point is connected to ground through a neutral grounding resistor (NGR), as shown in Fig. 3. If a phase-to-ground fault occurs, then the NGR will limit the voltage and the current. Mining regulations require that the value of the resistor be such as to limit the voltage that appears on portable equipment to 40 or 100V, depending on system voltage, and to limit the current that flows through it during a phase-to-ground fault to 25A. Limiting the fault current also eliminates an Arc-Flash that would be caused by a single phase-to-ground fault.
Neutral Grounding Resistor Is the Weak Link
All this is fine, but HRG systems can have problems as well. NGRs are located outdoors, and subject to corrosion and mechanical damage which can cause them to fail open circuit. This turns the HRG system into an ungrounded system, usually without anyone knowing it. There have been mining operations in which NGRs were found to have been open for long periods of time. And with an open NGR it is impossible to detect a ground fault with current transformers – a serious safety concern.
Clearly it’s important to know the condition of the NGR, both for safety reasons and because mining regulations require operators to test ground continuity from time to time. There are several different ways to monitor an NGR, but most of them are not satisfactory. Physically examining all the NGRs at a work site on a regular basis is impractical, because no matter how frequently the inspection is done, an NGR can fail in between inspections, and sometimes a fault is not visible to the eye.
Continuous NGR Monitoring Provides Safety and Compliance
Fortunately, continuous monitoring can be achieved electronically, by measuring the NGR’s resistance, the voltage across it, and the current through it. This is a complicated arrangement, but commercially available NGR monitors are available (Fig. 4) that automatically remove power if the system becomes ungrounded or alarm if continuity of service is required. Such monitors are easy to retrofit on existing HRG systems, and they can communicate their status to automatically provide the logs that MSHA requires. Most important, managers can be assured that ground faults will be detectable, transient overvoltages will be prevented, and that currents are safely below the level needed to form an Arc-Flash.
It is surprising how many aggregate operations do not adequately monitor their NGRs, considering that a small investment in technology can ensure that electrical safety equipment is working.
Jeff Glenney, P.E., is an engineer at Littelfuse.