Low-voltage breaker and tie interlocking

Short answer

In low voltage, interlocking between breakers prevents two sources from being paralleled when the system is not designed for it: two incomers and a tie, or the utility and a standby generator. It is done with a cable or rod mechanism between the breakers, often complemented by an electrical interlock with auxiliary contacts and an undervoltage release. The mechanical interlock is the one that depends neither on the control supply nor on how the breaker is closed.

Why low-voltage breakers are interlocked

In a low-voltage switchboard, interlocking between breakers almost always has a single purpose: to prevent two sources from being paralleled when the system is not designed for it. The typical situations are:

Two incomers and a tie (main-tie-main), with two transformers feeding two halves of a switchboard.
Utility and standby generator, when the transfer is open transition.
Two transformers that cannot run in parallel because of vector group, impedance or short-circuit level.

Paralleling two sources unintentionally has well-known consequences: circulating currents between transformers, a short-circuit level higher than the breakers and the switchboard can withstand, or a generator connected to the grid out of synchronism. The interlocking does not fix any of the three afterwards; it prevents them from happening.

Applicable standards

Low-voltage circuit breakers are covered by IEC 60947-2 and switchgear assemblies by IEC 61439-2. In North America, low-voltage power circuit breakers follow IEEE C37.13 and UL 1066, and molded-case breakers UL 489. The IEC 62271 series, which defines the interlocks of MV switchgear and disconnectors, covers voltages above 1 kV and does not apply here. That is why, in low voltage, interlocks are accessories each breaker manufacturer offers and the switchboard designer selects and integrates. Air circuit breaker families include two- and three-breaker interlocks in their catalogs.

Mechanical interlock

It links the breaker mechanisms with a steel cable or with rods. When one breaker is closed, the mechanical link blocks the closing mechanism of the others. The usual combinations are:

1 of 2: utility and generator, or two incomers without a tie. Only one can be closed.
1 of 3: three sources on a common bus. Only one can be closed.
2 of 3: two incomers and a tie. Any pair can be closed, but never all three. One manufacturer describes it, for example, as the interlock that allows two of three breakers to be closed at the same time, with the tie open or closed.

A cable allows the breakers to be apart inside the switchboard, within a length and bending limit set by the manufacturer; rods require adjacent or stacked breakers. With drawout breakers, the interlock acts with the breaker in the connected position, and it must be checked that racking one out to test does not release an improper close on the others.

2-of-3 interlock in a main-tie-main switchboard

How many of the three breakers are closed?
- None or oneAny of the others can close
- TwoThe third is mechanically blocked, whether it is an incomer or the tie

Electrical interlock

It is built with auxiliary contacts: the normally closed contact of one breaker is wired in series with the closing circuit of the other. If the first is closed, the second does not receive the close command. It is simple, but it only acts on electrical closing.

To cover manual closing, the undervoltage release is used: it is supplied through the normally closed contact of the other breaker. If the other one is closed, the release is de-energized and the breaker cannot close, by command or by hand, and if it was closed it opens. The drawback is that it depends on the auxiliary supply: if that supply fails, every breaker with an undervoltage release opens or cannot close, which can leave the switchboard out of service.

Why they are combined in low voltage

A low-voltage breaker has a mechanical close push button on its front that bypasses the control circuit. If only the electrical interlock by contacts exists, that button defeats it. The usual practice in switchboards with two sources is therefore the mechanical interlock as the main barrier and the electrical one as a complement, so the automatic transfer logic does not attempt a close the mechanism will reject.

When the transfer is closed transition, with a brief synchronized parallel between sources, the mechanical interlock cannot be used as is, because it would prevent the parallel. It is replaced by logic that limits the parallel time and checks synchronism.

Trapped key as an alternative

When the breakers are in different switchboards, too far apart for a cable, or come from different manufacturers, the mechanical interlock is done with a trapped key. In a main-tie-main, two keys are used for three locks: each breaker only closes with a key inserted, and since there are two, only two can be closed. The key also links the switchboard with the MV panel feeding each transformer, which a cable cannot do. Its limit is the control of copies.

The contactor case

In reversing starters, the forward and reverse contactors cannot close at the same time because they would short-circuit two phases. They are interlocked with a mechanical interlock between them and with crossed auxiliary contacts in their coils. It is the same principle on a smaller scale, and it is usually the first result when people search for electrical and mechanical interlocking.

Typical failures

An interlock cable that drifted out of adjustment or was cut during maintenance, and stops blocking without anyone noticing.
A breaker replaced by another one without the interlock accessory.
An auxiliary contact changed from normally closed to normally open during a modification.
A standby generator added to a switchboard without redoing the interlock with the utility supply.
An undervoltage release supplied from a source that is lost exactly when the transfer needs it.

How it is verified

Every forbidden close is attempted, by command and by hand, with every combination of closed breakers. With drawout breakers the test is repeated with each breaker in the connected and test positions. The undervoltage release is checked with loss of auxiliary supply, and the transfer logic is reviewed.

We help with your switchboard

Our team tests interlocks, breakers and transfer schemes of low-voltage switchboards in industry, substations and solar plants, in Colombia, or elsewhere on request, and designs the logic when there is on-site generation or closed transition transfer. See secondary engineering or contact us.

References

IEC 60947-2, Low-voltage switchgear and controlgear – Part 2: Circuit-breakers.
IEC 61439-2, Low-voltage switchgear and controlgear assemblies – Part 2: Power switchgear and controlgear assemblies.
IEEE C37.13, Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures.
ABB, SACE Emax 2 technical catalog, mechanical interlocks between breakers (as a manufacturer example).
Schneider Electric, Electrical Installation Guide, “Interlocks and conditioned operations”.

Frequently asked questions

Why is electrical interlocking not enough in low voltage?

Because a low-voltage power breaker can be closed with the mechanical push button on its front, bypassing the control circuit. The electrical interlock only acts on electrical closing, unless an undervoltage release is used, which in turn depends on the auxiliary supply. The mechanical interlock blocks closing in every case.

What is a 2-of-3 interlock?

It is the one used in switchboards with two incomers and a tie: of the three breakers, only two can be closed at the same time. Each half of the switchboard can be fed from its own incomer or from the other one through the tie, but the two incomers are never paralleled.

Does IEC 62271 apply to low-voltage breakers?

No. The IEC 62271 series covers switchgear above 1 kV. Low-voltage circuit breakers are covered by IEC 60947-2 and switchboards by IEC 61439-2, and in them interlocks are manufacturer accessories and part of the switchboard design.