Circuit Breaker Fails to Open: Causes and Diagnosis

Short answer

When a circuit breaker does not open on a trip order, the cause is almost always in the control circuit: an open coil or wiring, insufficient DC voltage, or a lockout from low SF6 density. Less often the problem is mechanical, from hardened lubricant or a jammed latch. The relay record and the coil current show where along the path the order was lost.

Symptom

A protection issues a trip order, but the breaker stays closed. Sometimes it shows because breaker failure 50BF operated and a whole bus went out of service; other times because the operator sent an open command from SCADA or from the panel and the equipment did not respond. In both cases the breaker is left in a dangerous condition: it is not known whether it will be able to interrupt the next fault.

The opening of a breaker depends on a short chain: relay output contact, wiring, 52a auxiliary contact, trip coil, mechanism latch and stored energy (springs, hydraulic or pneumatic). Any broken link prevents opening. Diagnosis consists of finding which link lost the order.

Possible causes, from most to least likely

Cause	Why it happens	What gives it away
Open trip circuit	Burned coil, loose terminal, cut wire or a 52a contact that does not close	74TC supervision alarm before the event; no coil current
Low or absent control voltage	Open control fuse or breaker, charger failure, discharged battery	DC system alarms; low voltage measured at the terminals
Lockout from low SF6 density or low operating pressure	Gas leak or loss of hydraulic or pneumatic pressure	Earlier first-stage alarm and active lockout signal
Jammed mechanism	Hardened lubricant, corrosion, stuck latch or coil armature	Coil current flowed, but opening did not occur or was very slow
The order did not leave the relay	Incomplete logic or trip matrix, test mode, damaged output contact	The relay recorded the pickup but not the output, or the output did not change state
A pole did not open	Single-pole operation with a pole failure or damaged mechanical linkage	Current still flowing in a single phase; pole discordance or 50BF operates

What to check first

Relay event record. Confirm that the trip order was issued, at what time and through which contact. If the relay did not issue it, the problem is in the logic or the output, not in the breaker.
Oscillography and coil current. Many relays and recorders show whether current flowed through the trip coil. With no current, the circuit was open or without voltage; with current, the problem is in the mechanism.
Earlier alarms. Check whether before the event there was an alarm from the trip circuit supervision, from low SF6 density, from discharged springs or from control voltage. Many opening failures announce themselves hours or days in advance.
DC system. Battery voltage, charger status, fuses and ground alarms in the substation DC system. IEC 62271-1 requires the opening coil to operate between 70 and 110 % of its rated DC voltage; below that range opening is no longer guaranteed.
State of the gas or pressure lockout. If the SF6 density monitor reached its second stage, the lockout operated by design.

Where the open order was lost

Did the relay record the trip output?
- No: Check the logic, the trip matrix, the test mode and the relay output contact
- Yes: Did current flow through the trip coil?
  - No: Was there control voltage at the trip circuit terminal block?
    - No: Check fuses, charger and battery bank of the DC system
    - Yes: Open circuit downstream: wiring, 52a contact, density lockout contact or open coil
  - Yes: Mechanical or single-pole failure: do not operate it again and test the breaker de-energized

How to confirm

With the breaker isolated and under test, the coil resistance and the continuity of each section of the trip circuit are measured, including the 52a contact.
Timing test with coil current recording. The shape of the current shows the travel of the armature and the moment the latch is released. An armature that is slow to move, or an opening time longer than in earlier tests, points to lubrication or wear. See the circuit breaker operating time test.
Minimum operating voltage. It is verified that the coil opens within the voltage range that the standard sets and the manufacturer declares.
SF6 density compensated to 20 °C and a test of the monitor, if the lockout was involved.
Mechanism inspection according to the manufacturer’s manual: latches, springs, dampers and linkage to the poles.

How to keep it from coming back

Supervise the trip circuit with the breaker both closed and open, and treat the 74TC alarm as urgent, not as just another notice.
In high and extra-high voltage, use two trip coils with separate circuits and separate DC supplies, each with its own supervision.
Maintain the mechanism at the interval the manufacturer indicates. A breaker that goes years without operating accumulates hardened lubricant; operating it on a schedule helps detect it in time.
Record the coil current at every maintenance to compare trends.
Respond to the SF6 first stage before it reaches the lockout.

What not to do

Do not repeat open orders over and over: a coil with no mechanism to release can burn out.
Do not open with the mechanical trip a breaker locked out by low density when it carries load or fault. Isolate by opening the upstream breakers.
Do not open disconnectors under load to take out the breaker that will not open.
Do not bypass the supervision or the lockout to “get by.” Those contacts exist to prevent a bigger failure.

What trip circuit supervision monitors and what it does not

A large share of breakers that do not open have the problem in the trip circuit and not in the mechanism. The causes are the coil without voltage, a loose terminal, a broken conductor, an open control fuse or miniature circuit breaker, or a relay output contact that does not close. Trip circuit supervision (function 74TC in ANSI nomenclature) exists to detect that condition before a trip is needed.

According to the description of an ABB supervision relay, the function continuously monitors the circuit, also with the breaker open. It detects loss of voltage, circuit interruption, contact degradation and increased resistance in the wiring and coils. Schemes pass a supervision current far below the coil operating current to test continuity. A Siemens relay of the 7PJ13 family, for example, uses a supervision current of 0.7 to 1.5 mA and alarms when the supply is lost or the circuit opens. The typical causes of a defective circuit that the sources cite are loose terminals, rodent damage to wiring and failure of the trip supply.

There are three points that help avoid over-trusting the supervision:

It must cover the breaker both open (before closing) and closed, so that the trip is assured in both states. A scheme that supervises only one of them leaves the other blind.
Classic schemes with a parallel relay do not detect a short-circuited coil and can alarm during a trip. Their coverage must be known before accepting them.
The supervision checks continuity, not the coil’s ability to move the mechanism. A coil with insufficient current because of low battery voltage or high-resistance wiring can pass the supervision and still fail to open.

On the breaker itself, the standard IEC 62271-100 applies, in its third edition of 2021 (with consolidated version 3.1 of August 2024). It covers three-phase AC circuit breakers for indoor or outdoor installation, at 50 Hz or 60 Hz, in systems above 1,000 V. It relies on IEC 62271-1:2017 for the common conditions. The voltage ranges of the opening devices and the behavior of the mechanism are read from those standards and from the manufacturer’s manual, not assumed.

When the breaker does not open, the 50BF must act as backup. If the breaker repeatedly refuses to open in local operations, the cause is usually in the mechanism, and the timing tests and the operating mechanism tests confirm it.

When to call us

If the breaker did not open and the 50BF operated, the first task is to find out whether the problem was in control or in the equipment. In the trip analysis we analyze the event record and the oscillography to locate the link that failed; we quote the analysis. If the breaker must be tested, we do circuit breaker testing: timing, coil current, minimum voltage and contact resistance, on site in Colombia, or elsewhere on request. Write to us.

References

IEC 62271-1, High-voltage switchgear and controlgear – Part 1: Common specifications.
IEC 62271-100, High-voltage switchgear and controlgear – Part 100: Alternating-current circuit-breakers.
IEEE C37.119, Guide for Breaker Failure Protection of Power Circuit Breakers.
Operation and maintenance manuals from the breaker manufacturer.

Frequently asked questions

Why did the breaker not open if the relay did issue the trip order?

Because the order did not reach the coil or the coil could not move the mechanism. The most common causes are an open trip circuit (coil, terminal, 52a contact), low control voltage and a lockout from low SF6 density. If current flowed through the coil and the breaker did not open, the problem is mechanical.

Which protection operates when the breaker does not open?

Breaker failure protection 50BF. If, after the trip order, current is still present once its set time has passed, it trips the adjacent breakers to isolate the fault, which takes a larger area out of service.

Can a breaker locked out by low SF6 density be opened manually?

It must not be opened under load or fault: below the lockout density the manufacturer no longer guarantees that the chamber will interrupt the current. It is isolated by opening the upstream breakers, and only then is the de-energized equipment worked on.