Shunt-Trip Circuit Fails to Trip at 125 VDC
A 125 VDC station battery with 60 lead-acid cells floats at 132 V and supplies the shunt-trip coils for a 15 kV metal-clad switchgear lineup. One feeder breaker fails to trip during a routine trip test. The trip circuit runs 350 ft one way in #10 AWG copper (1.24 Ω per 1000 ft at 75°C) from the battery panel to the breaker, through two terminal blocks and a 52a auxiliary contact. The shunt-trip coil nameplate reads 125 VDC, 55 Ω, with a minimum pickup of 80% of rated voltage. With the battery at 130 V, a high-impedance digital multimeter at the coil terminals reads 68 V while the trip command is held. The coil measures 54 Ω with a low-resistance ohmmeter. The 52a contact looks slightly pitted, and one terminal block screw is discolored but tight. A second identical feeder trips normally, with 128 V measured at its coil.
1. Calculate the coil voltage expected from copper resistance alone, then determine the unintended series resistance implied by the 68 V reading.
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The loop has 350 ft one way, so 700 ft of conductor. R_cu = 700 × 1.24/1000 = 0.87 Ω. With the 55 Ω coil, total R = 55.87 Ω. At 130 V, I = 130/55.87 = 2.33 A, and V_coil = 2.33 × 55 = 128 V. That is well above the 100 V pickup (80% of 125 V). The measured 68 V is therefore not explained by conductor resistance.
From the measurement, I = 68/54 = 1.26 A, so total circuit resistance is 130/1.26 = 103.2 Ω. Subtracting the 54 Ω coil and 0.87 Ω copper leaves about 48 Ω of unintended series resistance. That is the fault. The circuit is not marginal; the extra resistance is nearly equal to the coil resistance, which is why the coil cannot pick up.
2. Where is the extra resistance most likely located, and what test procedure confirms it?
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The 48 Ω is a high-resistance connection, not the coil. Candidates are the pitted 52a contact, the discolored terminal block, a corroded splice, or a loose fuse clip in the trip-supply circuit. Do not rely on an ohmmeter; it tests at low current and can read a few ohms through a connection that breaks down under load.
With the trip command held, measure the voltage drop across each segment: battery terminal to panel, panel to terminal block, across the 52a contact, and across each terminal. The segment with the largest drop is the culprit; a good connection should drop only millivolts at 1–2 A. The discolored terminal is a strong clue, but confirm with the drop test. Repair or replace the contact and terminal, then repeat the trip test at 100% and at reduced battery voltage (for example 105 V) to prove pickup margin.
3. Why can a high-impedance DMM mislead, and what acceptance criteria should replace a simple voltage reading?
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A high-impedance DMM (10 MΩ) draws almost no current, so with the circuit open it can read nearly full battery voltage through a high-resistance connection or even through capacitive coupling. The 130 V present reading at the coil does not prove the circuit can deliver current. Use a low-impedance meter or a loaded test (a 5 kΩ resistor across the probes), or better, measure while the coil is energized and current is flowing.
Acceptance is not voltage present; it is coil voltage at or above 80% of rated (100 V) with the trip command applied, plus a successful trip from the protective relay at minimum battery voltage. A trip-circuit monitor that only senses voltage can also be fooled; verify that it supervises current or uses a proven scheme.