Petone Relay Circuits

Petone Box has an Individual Function Switch panel controlling a relay interlocking. Planning for the new Petone signal cabin began in the late 1940s and it was commissioned in 1952.

The switches on the panel are free to move and the signals and points change only when the interlocking allows it. Approach and backlocking is provided on all signals. The Box was converted to switch in/out operation in 1975. A mixture of shelf and plug in relays are used.

It is likely that the Petone interlocking will be replaced towards the end of 2013.

circuit icon

(Above) A 1950s era hand drawn circuit diagram.

Circuit Symbols

If you are not familiar with relay circuits I recommend that you look at the circuit symbols page first because the terminology can be confusing.

Apparatus Naming Conventions

Relay circuits are easier to follow if you understand the Apparatus Naming Conventions.

Individual Function Switch Panel

I explain the concept of an Individual Function Switch panel.

Approach Locking - Signals

I explain the relay circuits used to implement the approach and back locking of signals.

Route Locking - Points

Sometimes points are route locked directly rather than indirectly through the signals. Route locking can lock points during a non-signalled move and start a timer. (Page under development).

Interlocking Points and Signals

I explain how points and signals are interlocked.

Switch In/Out

Petone was converted to Switch In/Out operation in 1975.


I explain the operation of the A-Lights at Petone.

Mechanical Latching Relays

1930s era Mechanical latching relays were used at Petone until October 2012. The are known to the maintainers as monkeys (short for monkey on a stick).

Magnetic Latching Relays

Petone also uses Magnetic Latching relays

Melling Branch Circuits

In 1983 most signalling and track circuits were removed from the Melling Branch. Since then trains have been signalled on a train on/train off basis. When there are non-standard moves the system is not foolproof and the signalman must pay careful attention to the train register.

Replacing The Monkeys

The monkey relays at Petone are a major source of faults. Over Labour weekend 20 - 22 October 2012 the monkeys were replaced with alternative circuits.

Home Signals

The 6 Up Outer Home and 77 Down Home From Main signals are approach locked but not backlocked. This means that when a train passes the signal and the lock stick drops the normal relay will pick when the switch is restored. This releases the interlocking. However the lock stick will not operate when the switch is moved to the reverse position because the backlock track is occupied. This prevents the signal from clearing and the out of correspondance indication is displayed. Points remain locked due to track circuits occupied.

This raises a question - why are the Home signals not backlocked?

Last Updated: Saturday October 31 2015

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