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INSTANTANEOUS CIRCUIT BREAKER SETTINGS FOR THE SHORT CIRCUIT PROTECTION OF THREE PHASE 480, 600 AND 1040V TRAILING CABLES
George Fesak, William Helfrich, William Vilcheck, David Deutsch U.S. Department of the Interior Mining Enforcement and Safety Administration Arlington, Virginia
ABSTRACT
Present Federal regulations which specify maximum instantaneous circuit breaker settings for the short-circuit protection of coal mine trailing cables are discussed. Characteristics of mine power systems which limit short-circuit current in three-phase trailing cables are analyzed and minimum expected short-circuit currents for three-phase 480, 600, and 1040V trailing cables are tabulated. New maximum instantaneous short-circuit currents and typical circuit breaker tolerances are proposed with emphasis on safety. Finally, a typical mine power systems are discussed and field tests cited.
INTRODUCTION
Trailing cables on electric face equipment in underground coal mines undergo more severe service than most other cables in
industrial applications. The normal operation of a unit of self-propelled mining equipment subjects its trailing cable to extreme tensile
forces, severe abrasion, and frequent flexing, twisting and crushing. As a result of this severe usage, electrical faults in trailing cables
occur much more frequently then electrical faults in cables and wiring in stationary industrial installations.
Of the various faults which occur in trailing cables, the short circuit has proven to be one of the most hazardous.
The energy expended in a short circuit in a trailing cable is capable of igniting loose coal and coal dust on the mine floor, as well as loose coal,
coal dust, hydraulic oil and other combustible materials onboard a mining machine. Between 1952 and 1969, the Bureau of Mines investigated 265 mine
fires caused by short circuits in trailing cables. These mine fires were responsible for 13 deaths and 50 injuries.
If the arc from a short circuit is not contained within the trailing cable jacket, and the short circuit occurs
where an explosive mixture of methane and air is present an ignition is likely to occur. In "Electrical Hazards in Underground Bituminous Coal Mine"
[1], Mason reports that during the period 1952-1968, 21 methane ignitions and explosions were caused by electrical faults in trailing cables. These
ignitions and explosions resulted in nine fatalities and 18 injuries.
Even if a short circuit in a trailing cable does not cause a fire or a methane ignition, the energy delivered into
the fault can cause combustion of the cable at or near the location of the short circuit, there exists the possibility of flash burns to the hands
and eyes.
The frequency of short circuits in trailing cables, coupled with the potential hazards associated with their
occurrence, makes adequate trailing cable short-circuit protection extremely important. The importance of trailing cable short-circuit protection
has been recognized for many years, and requirements for such protection have been included in Federal standards for permissible electric face
equipment since Bureau of Mines Schedule 2C [2] was written in 1930. However, it was not until the Federal Coal Mine Health and Safety Act of 1969
was enacted that short-circuit protection for all trailing cables was required by Federal statute.
Section 306(b) of the Act requires that each trailing cable be provided with short-circuit protection by means
of an automatic circuit breaker or other no less effective device approved by the Secretary of the Interior, but does not specify the circuit breaker
type or maximum setting. These requirements, however, were developed and promulgated in accordance with the authority and responsibility given to
the Secretary of the Interior by the Act and are included in the "Mandatory Safety Standards for Underground Coal Mines" (Title 30, Code of Federal
Regulations, Part 75).
Section 75.601-1, 30 CFR 75, specifies the maximum allowable instantaneous settings for circuit breakers used
to provide short-circuit protection for trailing cables. These settings were determined by applying a 50% safety factor to the line-to-line
short-circuit current calculated by assuming an infinite capacity 250V dc power source and 500 ft. of 2-conductor trailing cable. The 50% safety
factor was included to account for power system impedance, voltage dips, circuit breaker tolerances, etc. In addition, a maximum circuit breaker
setting of 2500 A was established. Section 75.601 -1, 30 CFR 75, also contains provisions for allowing higher circuit breaker settings when special
applications justify them.
Since the implementation of the Federal Coal Mine Health and Safety Act of 1969, there has been a significant
reduction in the number of mine fires and methane ignitions caused by short circuits in trailing cables. Since 1970, there have been only nine
mine fires caused by short circuits in trailing cables. These fires did not result in any fatalities or injuries. During the same period there
were no methane ignitions caused by short circuits in trailing cables. It is apparent that improvements in trailing cable electrical protection
as well as improvements in trailing cable splicing, mine ventilation and fire protection brought about by the Act have significantly reduced the
number of severity of trailing cable short circuits. Nevertheless, 1972 and 1973 accident data reported in [1] indicate that electrical faults in
trailing cables continue to result in a significant number of serious flash burn and electrical burn injuries to miners' hands and eyes.
Although the maximum circuit breaker settings specified in Section 75.601-1, 30 CFR 75, are based on the
calculated short circuit current in 250V dc trailing cables, the settings are applied to all trailing cables, including three-phase trailing
cables energized at 480, 600, and 1040V. The significant reduction in the frequency of mine fires and methane ignitions caused by short circuits
in trailing cables indicates that the settings specified in Section 75.601-1, 30 CFR 75, generally provide adequate short-circuit protection for
three-phase trailing cables. Nevertheless, short-circuit surveys conducted by MESA electrical engineers have shown that in certain instances these
settings do not provide an adequate margin of safety for three-phase trailing cables.
There are other cases in which the settings specified in Section 75.601 -1, 30 CFR 75, are lower than necessary
to provide adequate short-circuit protection for three-phase 480, 600, and 1040 V trailing cables. In several of these cases it has been necessary
to raise circuit breaker settings to eliminate nuisance tripping as a result of peak machine inrush or operating current.
This paper will attempt to meet the need for a new table of maximum instantaneous circuit breaker settings for
the short-circuit protection of three-phase 480, 600, and 1040 V trailing cables based on an analysis of the minimum expected short-circuit current
in three-phase trailing cables and the characteristics of the circuit breakers commonly used to provide trailing cable short-circuit protection.
The paper will also discuss conditions under which the maximum allowable circuit breaker settings should be reduced to afford an adequate margin of
safety as well as the conditions under which the maximum settings may be raised without sacrificing safety.
MINIMUM EXPECTED SHORT-CIRCUIT CURRENT
The requirement for instantaneous short-circuit protection of trailing cables places several
constraints on the selection of maximum instantaneous circuit breaker settings. Safety considerations demand that the circuit breaker trip whenever
the minimum value of short-circuit current flows in the trailing cable. Consequently, the maximum specified circuit breaker setting must take into
account the circuit breaker tolerance as well as the many factors which limit short-circuit current, including fault type and location, circuit
voltage, power system impedance, section transformer impedance and trailing cable impedance.
Safety considerations cannot be compromised. However, the short operating time of an instantaneous trip circuit
breaker requires that the circuit breaker be set to trip at a current greater than the peak starting and/or operating current of the machine
connected to the trailing cable. Otherwise, nuisance circuit breaker tripping would require a larger trailing cable than necessary for ampacity
considerations alone.
In view of the above, any tabulation of maximum allowable circuit breaker settings should take into account
sufficient parameters to assure that for the majority of situations encountered, the specified settings will provide the necessary protection without
being overly restrictive. On the other hand, the tabulation should be presented in a simple and concise manner so that it is as easy as possible
to use. Obviously, a tabulation of maximum circuit breaker settings would lose its usefulness if it was necessary to conduct a short-circuit survey
of the mine power system to determine each circuit breaker setting.
Calculation of Minimum Expected Short-Circuit Current
Calculations to determine minimum expected short-circuit current differ from the more common calculations to determine circuit breaker interrupting current requirements. In the latter case, the bolted fault condition yielding maximum current flow (usually the three-phase fault) is used as the basis for the calculation. The fault location yielding highest short-circuit current is chosen. In addition, the fault current contribution of the motors connected to the power system is added to the fault current delivered by the power system. However, when calculating minimum expected short-circuit current, the fault location and fault condition yielding minimum current flow must be used as the basis for the calculation.
Phase-to-Phase Faults
Of the variety of faults that can occur in a three-phase trailing cable, the phase-to-ground
fault results in the lowest current flow, since this current is limited by a neutral grounding resistor to 25 A or less in accordance with
the requirements of Section 75.901, 30 CFR 75. In addition, Section 75.900, 30 CFR 75, requires that all low- and medium-voltage underground
three-phase circuits be provided with ground-fault protection. However, since ground-fault protection is provided by separate devices sensitive
to the low magnitude of ground-fault current, it is not necessary for the circuit breaker setting to be based on ground-fault current flow.
Other than the phase-to-ground fault, the phase-to-phase fault yields the lowest current flow. Consequently, it is the minimum expected phase-to-phase
short-circuit current that must be used to determine maximum circuit breaker settings for trailing cable short-circuit protection.
The calculation of phase-to-phase fault current in three-phase circuits is treated extensively elsewhere;
therefore, only the general equation is presented here.
where IФФ – phase-to-phase fault current;
EФФ – phase-to-phase voltage;
KA – arcing fault factor;
Z1 – total positive sequence impedance;
and Z2 – total negative sequence impedance.
It should be pointed out that an arcing fault factor (KA) has been applied to the traditional
equation for bolted phase-to-phase fault current to account for reduced fault current due to the impedance of an arcing fault.
The negative sequence impedance of a static circuit element (transformer or cable) is equal to the element's
positive sequence impedance (Z2=Z1); however, the positive and negative sequence impedances of a dynamic circuit element
(motor or generator)
do differ. The difference becomes significant only when the fault is located close to the source generator. When the mine power system is supplied
from a utility, the difference between the positive and negative sequence impedance and, in fact, the total impedance of the utility generators
is insignificant. Consequently, one can accurately assume that the total negative sequence impedance is equal to the total positive sequence
impedance. This allows equation (1) to be further simplified.
In instances where the mine power system is supplied power from an onsite generator, a special analysis, involving the transient or subtransient impedance and the negative sequence impedance of the generator, must be made to calculate phase-to-phase fault current.
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