Pole discrepancy protection (ANSI 62PD) — when not all poles of a circuit breaker switch together

The guide on circuit-breaker failure protection (ANSI 50BF) covers the scenario where a circuit breaker receives a trip or close command but fails to operate at all. There is, however, a subtler failure mode for circuit breakers with independently operated poles (each phase driven by its own mechanism, common at higher voltage levels): one or two poles do switch, but not simultaneously with the other poles. This is called pole discrepancy, and it is detected by pole discrepancy protection, ANSI 62PD.

The mechanism: mechanical or hydraulic asynchrony between poles

In a three-pole circuit breaker with a common operating mechanism (one mechanism for all three poles via a coupled shaft), simultaneous switching of all three poles is mechanically guaranteed, apart from some small, inherent scatter. In breakers with independently operated poles — each phase with its own spring, hydraulic, or pneumatic operating mechanism, often used at higher ratings or to allow single-pole reclosing after a single-phase fault on an overhead line — a mechanical or hydraulic defect in one pole mechanism can cause that one pole to close late, open late, or even fail to respond at all while the other two poles switch normally.

Why this is dangerous: unbalanced phase loading and overvoltage

A prolonged pole discrepancy — for example two poles closed and one pole open, or the reverse — produces a strongly unbalanced three-phase condition:

In motors and generators, a significant negative-sequence current arises, with the associated rotor heating also covered in the guide on negative-sequence protection (ANSI 46) — but here caused by a switching fault rather than a network fault.
With an isolated or high-resistance-earthed neutral, an open phase combined with the remaining two closed phases still connected through load or transformer windings can shift the neutral point and cause a temporary overvoltage on the healthy phases.
When a transformer is single-pole disconnected on one side while the other side remains fully connected, an unusual magnetizing condition can arise, with associated noise and vibration effects.

A prolonged pole discrepancy is therefore not a harmless transient condition, but a situation that must be terminated quickly for both equipment and network stability.

How the protection works: auxiliary contacts and time delay

Pole discrepancy protection typically uses the auxiliary contacts of each individual pole, which report the actual open/closed status of that pole — not the switching commands themselves, but the real mechanical end position. A logic circuit (or numerical relay) compares the status of the three poles:

If all three poles are in the same state (all open or all closed), there is no discrepancy.
If the state of one or two poles differs from the other(s), a time delay starts, typically on the order of a few hundred milliseconds to a couple of seconds.
If the discrepant condition persists after this delay elapses, the protection issues a three-pole trip command to the same circuit breaker (to open any remaining closed pole or poles) and usually also a start signal to circuit-breaker failure protection (50BF) in case the breaker itself no longer responds reliably.

Note: The 62PD time delay must be long enough to ride through normal, brief pole asynchrony during a planned switching operation (the small, inherent scatter between independent pole mechanisms during a normal three-pole switching command), but short enough to limit the risks described above.

Practical relevance

For circuit breakers with independently operated poles — often applied precisely because single-pole reclosing after a single-phase fault on an overhead line is desired, as covered in the guide on automatic reclosing (ANSI 79) — 62PD protection must be present precisely because the capability for single-pole switching also increases the risk of a prolonged, unwanted single-pole condition. For three-pole breakers with a common operating mechanism, 62PD is typically not needed, since the failure mechanism it detects cannot mechanically occur there.

Common mistakes

Confusing pole discrepancy protection (62PD) with circuit-breaker failure protection (50BF) — 50BF detects that a breaker fails to respond to a switching command at all; 62PD detects that the poles of a breaker do not switch simultaneously, even when the breaker does (partially) respond.
Applying 62PD to three-pole breakers with a common operating mechanism where the failure mechanism cannot mechanically occur, without recognizing that the function is redundant there.
Setting too short a time delay, causing normal, brief pole asynchrony during a planned three-pole switching operation to already produce an unwarranted trip command.
Forgetting that 62PD must issue a start signal to 50BF, so that a prolonged pole discrepancy at a breaker that no longer fully responds does not lead to a timely trip of the upstream breaker.