Circuit Breaker Trip Due To Control Circuit Ground Fault

Control circuit for medium and high voltage circuit breakers have protective relay contacts to trip and close circuit breaker. Control circuit can be powered from AC or DC control voltage. In this article possibilities of nuisance tripping of circuit breaker [CB] due to accidental intermittent or permanent ground fault [GF] is discussed. Intermittent control ground fault nuisance trip is especially hard to identify and troubleshoot. Intermittent control circuit ground fault can cause CB tripping or closing unless suitable precautions are taken. Fortunately, these can only take place under rare conditions.

Read: Stray voltage: How does it originate?

For AC systems, risk arises primarily when phase and neutral is incorrectly wired to trip or close contacts. For DC systems, risk arises due to static charge stored in the control wire stray capacitance to ground. For some large indoor and many outdoor substation yards, control cabling can be hundreds or even thousands of meters long with large stray capacitance.

Figure 1: Substation yard control cabling can be very long

The phenomenon discussed in this article can cause breaker tripping and/or closing. However, closing operation requires significantly more energy [VA] than tripping and hence any capacitive discharge nuisance trips [with DC control voltage] are going to be less likely unless interposing electromechanical [ice cube] relays are in the close circuit. Accidental closing operations due to ground fault in AC system due to incorrect wiring (discussed below) is a possibility.

Read: Circuit breaker anti pump relay

AC Control Voltage

When AC control voltage is used, circuit below can misoperate if there is an accidental ground at the location shown. This circuit will work perfectly under normal circumstances except when circuit has ground fault. Note that this layout is not a common practice but is definitely possible if trip circuit is incorrectly wired as shown where the neutral is switched and not the phase. A similar risk exists for close coil if wired incorrectly.

Figure 2: Trip circuit that can misoperate during ground fault

This method of control circuit malfunction is a serious and permanent risk unlike the DC case where the amount of stray capacitance plays a critical role. Solution will be to always connect trip contact to hot or phase lead and not neutral.

Read: What is ghost or phantom voltage?

In a correctly wired AC system connected phase-neutral, an accidental ground fault will cause fault current to flow leading to protective device clearing. Ground fault will not cause nuisance trip or close with AC control voltage when correctly wired.

Read: Incorrect motor control circuit wiring

DC Control Voltage- Possibility# 1

Consider circuit shown in figure 3 where station DC battery is center point grounded. Capacitances shown are the stray capacitances of control cables. When there is a ground fault at the location shown, charge stored in the wire capacitance will discharge through trip coil which may cause nuisance trip if enough stored energy to magnetize the coil is present.

Figure 3: Accidental fault cause stray capacitance to discharge through trip coil

The amount of capacitance required to activate the trip or close coil will be large meaning the substation control wiring has to be very long. If the trip coil is an auxiliary relay such as ‘ice cube’ interposing relay, then not much energy is required to magnetize the relay and hence close the trip contact. See figure 6, 7.

Read: Voltage swell due to line to ground fault

DC Control Voltage- Possibility# 2

Consider circuit shown in figure 4 where station DC battery is connected to trip coil through a normally open [NO] trip contact. Capacitance C_T is the stray capacitance of control cabling between trip contact and trip coil. When there is a fault at the location shown, charge stored in wire capacitance will discharge through trip coil possibly causing nuisance trip.

This method of control circuit malfunction is only a risk when DC wire is very long. In most cases the amount of capacitance required to activate the trip coil will be large meaning the substation control wiring has to be very long. If the trip coil is a small ‘ice cube’ interposing relay, then not much energy is required to magnetize the relay and hence close the trip circuit. See figure 6, 7.

Read: CT failure due to capacitor bank discharge current

Figure 5: Electromechanical [Ice cube] relay and shunt trip coil

An experiment was done to find out the amount of stray capacitance needed to energize relay contacts and breaker trip coil. Standard commercially available relay and a low voltage circuit breaker shunt trip coil is used for this test. Note that this is not representative of all relays and trip coils. Results will vary depending on relay vendor and coil characteristics. A smaller relay might turn on with much smaller stray capacitance and vice versa.

Figure 6: Capacitance need to activate sample relay and trip coil

Notice from figure 6 that as the control voltage is increased from 24VDC to 125VDC, the amount of stray capacitance needed to energize relay contacts drops significantly. Energy stored in a capacitor is ½ CV² meaning higher control voltage translates in to larger stored energy.

Figure 7: 24VDC relay is transiently energized by 23.5uF of stray capacitance

Depending on the amount of energy stored in stray capacitance, trip coil can be transiently activated for a duration from few ms to hundreds of ms. Control circuit capacitance is affected by circuit length, thickness (gauge) of cable used, type of cable insulation and method of installation etc. Figure 7 shows waveform of 24VDC relay that was accidently energized by discharge of stray wire capacitance. Depending on how trip circuit is wired and how sensitive the trip circuit is; this transient energization of relay coil might be enough to open the circuit breaker.

Read: Capacitor bank discharge methods

Summary

When AC control voltage is used, ensure that trip or close contacts switch the phase circuit and not neutral. If neutral circuit is switched, an accidental ground between trip/close contact and coil will activate the coil and lead to nuisance trip or close. This problem can easily be avoided by following correct wiring practice.

With DC control voltage, when sensitive trip or close contacts exist, it is recommended to limit capacitance on the DC control wiring to prevent unwanted trip or close operation during accidental control circuit ground fault. Good thing is that length of cable needed to produce required stray wire-ground capacitance that can activate typical trip, close or relay coils is very long, usually thousands of meters. For this reason, smaller indoor substations are at low risk and large outdoor substations with multiple long control cable runs to the yard will have higher risk. Use of small electromechanical interposing [ice cube] relays also bring additional risk as very less energy is required to pull the coil and hence close the contacts. Higher the station DC control voltage, larger the stored capacitive energy and greater the risk for nuisance trip. Substations operating with 250VDC, 125VDC will have higher risk compared to those operating with 48VDC or 24VDC.