Parkend Exchange NU Tone Circuit
On any spare level the first group selector drives into the bank and tests into the battery condition on the NU tone circuit P wire and then extends the calling loop to operate the A relay. A1 operates the B relay which then returns an earth on the P wire to hold the connection. B is made slow to release so that should the caller continue to dial, then B will continue to hold the P wire and not release during the dial pulses.
Relay B also connects NU tone to the third A relay winding which transmits the tone to the caller via the transformer action within the relay coil.
When the caller clears, A releases which in turn releases B which removes the earth from the P wire thus releasing the connection.