Ensuring long life for electrical equipment
Resistance grounding systems have many advantages over solidly grounded systems including arc-flash hazard reduction, limiting mechanical and thermal damage associated with faults, and controlling transient overvoltages.
By Ed Hamilton and Michael D. Seal, P.E., General Electric
Losing plant power means downtime for critical equipment, loss of productivity and possibly loss of revenue. However proper grounding and effective maintenance procedures can help your facility avoid this drastic situation.
Using resistance grounding systems
A key action to help ensure the reliability of your electrical system is conducting appropriate grounding. Resistance grounding systems have many advantages over solidly grounded systems including arc-flash hazard reduction, limiting mechanical and thermal damage associated with faults, and controlling transient overvoltages. High resistance grounding systems may also be employed to maintain service continuity and assist with locating the source of a fault.
Generally, there are two types of resistors used to tie an electrical system's neutral to ground: low resistance and high resistance. Ground fault current flowing through either type of resistor if a single phase faults to ground, increases the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation and surge arrestor ratings must be based on line-to-line voltage. This temporary increase in phase-to-ground voltage should also be considered when selecting two- and three-pole breakers installed on resistance grounded low voltage systems. Many 480/277-V three-pole breakers, for example, carry single-pole interrupting ratings that are based on 277 V phase-to-ground. Once the phase-to-ground voltage increases to 480 V, the breaker's performance is not guaranteed.
The increase in phase-to-ground voltage associated with ground fault currents also precludes the connection of line-to-neutral loads directly to the system. If line-to-neutral loads such as 277-V lighting are present, they must be served by a solidly grounded system. This can be achieved with an isolation transformer that has a three-phase delta primary and a three-phase, four-wire, wye secondary.
Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with phase-to-phase faults, but both systems significantly reduce or essentially eliminate the arc-flash hazards associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses and reduce thermal damage to electrical equipment, circuits and apparatus carrying faulted current.
Neutral grounding resistors (NGRs) limit the fault current when one phase of the system shorts or arcs to ground. In the event that a ground fault condition exists, the NGR typically limits the current to 200-400 A, though most resistor manufacturers label any resistor that limits the current to 25 A or greater as low resistance. A particular resistor may be specified as 2,400 V L-N, 400 A, 10 seconds, meaning that the impedance of the resistor is such that 2,400 V applied across it will result in 400 A of current through it, and that the unit can carry this current for only 10 seconds before overheating.
As a rule of thumb, NGRs are designed with a continuous current rating equal to approximately 10% of its rated current. A unit rated 400 A for 10 seconds can carry 40 A continuously. In order to prevent the NGR from overheating, overcurrent protective devices must be designed to trip before the resistor's damage curve is breached.
High resistance grounding (HRG) systems limit the fault current when one phase of the system shorts or arcs to ground, but at lower levels than low resistance systems. In the event that a ground fault condition exists, the HRG typically limits the current to 5-10 A, though most resistor manufacturers label any resistor that limits the current to 25 A or less as high resistance. HRGs are continuous current rated, so the description of a particular unit does not include a time rating.
Unlike NGRs, ground fault current flowing through an HRG is usually not of significant magnitude to result in the operation of an overcurrent device. Since the ground fault current is not interrupted, a ground fault detection system must be installed. These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodically opens and closes).
When the contactor is open, ground fault current flows through the entire resistor. When the contactor is closed a portion of the resistor is bypassed resulting in slightly lower resistance and slightly higher ground fault current. A handheld pulsing current detector can then be used to track the ground fault to its source.
To avoid transient over-voltages, an HRG resistor must be sized so that the amount of ground fault current the unit will allow to flow exceeds the electrical system's charging current. The charging current for an electrical distribution system can be measured or estimated. As a rule of thumb, charging current is estimated at 1 A per 2,000 kVA of system capacity for low voltage systems and 2 A per 2,000 kVA of system capacity at 4.16 kV. These estimated charging currents increase if surge suppressors are present.
Each set of suppressors installed on a low voltage system results in approximately 0.5 A of additional charging current; each set of suppressors installed on a 4.16 kV system adds 1.5 A of additional charging current. A system with 3,000 kVA of capacity at 480 V would have an estimated charging current of 1.5 A. Add one set of surge suppressors and the total charging current increases by 0.5 A to 2.0 A. A standard 5 A resistor could be used on this system. Most resistor manufacturers publish detailed estimation tables that can be used to more closely estimate an electrical system's charging current.
Maintaining the backbone of a facility
Another key to prolonging electrical equipment is maintenance. With scheduled preventive maintenance, and upgrades to your electrical systems, you can bring your facility up to the most recent regulatory standards such as the NFPA 70E guidelines for improving arc flash protection in the workplace. Secondly, you can help reduce operating costs by minimizing unplanned equipment downtime and improving energy efficiency.
Electrical distribution equipment - especially switchgear - is the backbone of any plant or facility. Without a reliable power distribution and electrical fault protection system, the machinery required to manufacture products cannot produce. Therefore, the health of an organization can depend on the health of this electrical backbone.
The vertebrae of this backbone are the circuit breakers that are constantly looking for faults to interrupt, isolating the risk of a serious incident. Facility owners should regularly ensure their circuit breakers are operating successfully, and recondition or replace them when necessary. Upgrades to circuit breakers are available in the form of trip units that provide enhanced protection and access to modern communication protocols to improve arc flash protection and reduce operating costs respectively.
Maintenance is a very critical part of the flash hazard issue. Inadequate maintenance can cause unintentional time delays in the clearing of these devices during a short circuit condition. For example, if a low-voltage power circuit breaker has not been operated or maintained for several years and the lubrication has become sticky or hardened, the circuit breaker could take several additional cycles, seconds, minutes or longer to clear a fault condition.
If electrical protective devices such as circuit breakers and relays are not maintained on at least an annual basis, the likelihood of failure significantly increases.
No doubt breakers exist in facilities with old trip units. And many of them still work just fine. But eventually, just fine will not be good enough, often with the horrid realization that old trip units are prohibiting the operation of critical equipment. One bad vertebra - one bad trip unit - is all it takes to effectively paralyze an electrical system and cripple an organization. Maintenance of the circuit breakers generally consists of keeping them clean and properly lubricated.
Within these same facilities is equipment, acting as a central nervous system that provides control of motors, either as individual motor starters, or combined with a motor control center. Depending on the application, these devices can operate many times a day, causing mechanical and electrical wear. Therefore, regular maintenance and upgrades are recommended. Motor controls can be replaced as a preventive measure, and motor control centers can be upgraded with new buckets that provide relays with enhanced motor management for better protection and communications.
Motor controls can also be upgraded to improve energy usage by adding drives and soft starters to allow motors to run more efficiently and reduce mechanical stresses during the startup process.
An excellent source for appropriate preventive maintenance of your electrical equipment is the National Fire Protection Assn.'s (NFPA) 70B, "Recommended Practice for Electrical Equipment Maintenance." NFPA 70B offers four basic steps for effective PM:
1. Compile a list of all plant equipment and systems
2. Determine which equipment and systems are most critical and most important
3. Develop a system for assessing what needs to be accomplished
4. Train in-house staff to complete the scheduled work or hire a group for the needed services.
Qualified technicians trained in the use of the test equipment, safe work practices and electrical safety must be the only people performing electrical testing and maintenance. Only work on offline equipment that has been de-energized, and observe proper lockout/tagout (LOTO) is practices to minimize the possibility of energizing equipment inadvertently.
Whether using preventive maintenance, Reliability Centered Maintenance, Condition Based Maintenance or a combination of these, it's critical that maintenance be conducted on a regularly scheduled basis - at least once a year, every year. A scheduled maintenance program decreases the need for emergency maintenance, saving your facility costly downtime.
How often should you maintain your electrical systems? There is not a rule of thumb, but you should - at the very least - test your electrical equipment annually. If your facility has a shutdown period, schedule your electrical system maintenance at this time.
It can be tempting to disregard conducting proper testing and maintenance of electrical equipment. It might seem very difficult to argue with a "if it isn't broke don't fix it" approach to operations - especially during challenging economic times. But, if you compare how much it would really cost if you don't test, maintain and/or upgrade to how much it would cost if your facility experiences an outage due to faulty equipment, the financial decision becomes clear. Examining the real costs of your electrical system can help decision makers avoid expensive downtime.
Ed Hamilton is marketing manager for Industrial and OEM, and Michael D. Seal is a senior specification engineer, both for General Electric.
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