Mitigating arc flash hazards in medium-voltage switchgear
Maintenance mode on solid state protective relays: A maintenance switch is now available in most medium-voltage circuit breakers as a means of temporarily adjusting the settings of the solid state protective device during scheduled maintenance such that arcing faults are cleared without delay, while still maintaining the desired settings for coordination with downstream protective devices. Figure 4 shows the application and the benefit of a maintenance switch in 4.16 kV switchgear. Figure 4 A shows the single-line diagram of the switchgear. Figure 4 B shows the time-current curves of the main and the feeder breaker relays. The calculated arcing fault current is 8.44 kA for a bus fault. The fault is cleared by the main breaker in 1.303 sec (including the breaker time), the incident energy is 12 cal/cm2, and the HRC level is 3.
When the maintenance switch is engaged, the main breaker relay’s instantaneous setting is reduced from 80 (16,000 A) to 30 (6,000 A), below the expected arcing fault current. The arcing fault will now be cleared in 0.015 sec, the incident energy is reduced to 1.2 cal/cm2, and the HRC level is reduced from 3 to 1.
While using the maintenance switch, plant supervisors must enforce an error-proof method of ensuring that the maintenance switch is disengaged after the scheduled maintenance work is completed. Otherwise, there will be nuisance tripping of the main breaker.
Arc flash protective relays: Light emitted by the arc can be used to detect an arcing fault instead of current sensing. This is the principle of operation of arc flash protective relays now being marketed by some companies in the U.S. The result is the same as that of the maintenance switch except that no human action is necessary. Arcing inside the switchgear enclosure is detected by either a photoelectric receptor or a length of fiber-optic cable. The input is given to a single-function or a multifunction electronic protective relay, which can trigger instantaneous tripping of the breaker. This method is independent of the magnitude of the arcing fault current and can detect arcing in the early stage of its development. One company claims that the detection takes place in 1.0 msec. These relays have not gained wide acceptance yet, but they surely present a better way of detecting arcing and immediate tripping than current sensing.
Arc-resistant switchgear: In extreme cases, severe arcing in enclosed equipment can cause tremendous pressure buildup and may result in an explosion. The explosion will relieve the pressure buildup but will not quench or terminate the arc, which will proceed to cause thermal damage to the bus bars and enclosures until it is cleared by circuit breakers. This is the most probable scenario that resulted in several low-voltage and medium-voltage switchgear being completely gutted by internal arcing. Arc resistant switchgear is available that is structurally strong and has means of relieving the pressure buildup. The means consist of louvers and vents in the back of the enclosure, away from the operators, to exhaust the rapidly expanding air.
There are many environments in which the extra expense of the arc resistant switchgear is justifiable. In many industries, the additional cost is much lower than the cost of repair, downtime, compensations, and litigation.
Crowbar methods: A radically different method of dealing with arcing faults is what is known as the “crowbar” method. This method is well known in Europe and is recognized as a viable method in medium-voltage switchgear by the International Electrotechnical Commission Standard 62271-200. Unfortunately, no U.S. standard has yet been developed as a guideline for the application of this method. Essentially, the crowbar method consists of high-speed detection of arcing, intentional creation of a 3-phase bolted fault, and clearing of the bolted fault by the circuit breaker. The bolted fault is created by a grounding switch. The circuit voltage is brought to zero and the arc collapses. In the U.S., this method is marketed as a high-speed grounding switch (see Figure 5). Arcing is detected by an optical sensor. An electronic relay energizes an actuator, which closes the 3-phase grounding switch, thus creating a 3-phase bolted fault. The bolted fault is sensed by the system protective relaying and the circuit breaker is tripped. Another company is marketing a scheme in which, instead of creating a bolted fault, a second arc is created inside a confined and mechanically strong drum-like enclosure. This second arc being parallel with the fault arc serves the same purpose as the bolted short-circuit.
Remote operating panels: Safety of personnel from arc flash hazards can be ensured by providing remote operating panels from which all manual operation of the switchgear can be performed. The remote panels must be located at a safe distance from the switchgear or in a separate room. If space is available for the remote panels, the equipment itself is not expensive. All circuit breakers in the switchgear must be electrically operated. In addition, a motor-operated draw-out mechanism must be provided. All breaker control switches, auto/manual switches, indicator lamps, ammeter and voltmeter switches, meters, and an operator interface terminal can be installed in the remote operator panel (see Figure 6).
Calculating arcing faults
The following equations are used in calculating the arcing fault current:
For system voltage under 1 kV:
lg(Ia) = K + 0.662 lg(Ibf) + 0.0966 V + 0.000526 G + 0.5588 V (lg Ibf) – 0.00304 G (lg Ibf)
lg = the log10 (logarithm to the base 10)
Ia = arcing current, kA
K = -0.153 or open air arcs; -0.097 for arcs-in-a-box
Ibf = bolted three-phase available short-circuit current (symmetrical rms), kA
V = system voltage, kV
G = conductor gap, mm
For system voltage greater than or equal to 1 kV:
lg (Ia) = 0.00402 + 0.983 lg (Ibf)
The incident energy E is calculated using the following equation:
E = 4.184 Cf En (t/0.2) ( 610x/Dx)
E = incident energy, J/cm2
Cf = calculation factor
= 1.0 for voltages above 1 kV
= 1.5 for voltages at or below 1 kV
En = incident energy normalized
t = arcing time, sec
x = distance exponent
D = working distance, mm
The normalized incident energy is given by the following equation:
lg En = k1 + k2 + 1.081 lg(Ia) + 0.0011 G
In these equations, the values of G and the exponent x depend on the voltage and the type of equipment. For example, for 480-V switchgear, G = 32 mm and x = 1.473. For other voltages and other equipment, Table D.7.2 of IEEE Std. 1584 gives the values of G and x.
Source: IEEE Std. 1584-2002 IEEE Guide for Performing Arc-Flash Hazard Calculations
Syed M. Peeran is a senior electrical engineer at CDM Smith. He has more than 20 years of experience in the design of electrical distribution systems. For several years, he was an adjunct professor at Northeastern University, Boston, and is a member of the Consulting-Specifying Engineer editorial advisory board.