The Public Address system at Norchard is normally switched on for use by dialling 6 on the Strowger exchange. This operates the A relay and then the B relay in an exchange to PA interface relay set. The B relay sends out a battery earth on the MO wire which in turn switches on the mains to the PA rack.

When the PA rack switches on it returns a 24 volt signal to the level 6 equipment to operate relay PWR. PWR1 returns ring tone to the caller, though if the rack does not turn on, PWR2 would return NU tone instead. When the caller clears and removes the battery from the MO wire the mains control equipment holds and maintains the supply to the PA rack.

Should there be a failure to switch on, then a separate switch is provided outside the locked exchange room. This has a timer which provides a mains supply direct to the mains control unit for a period of three hours. The three hour switch unit is the one to the left of the units mounted outside the exchange.

The orange lamp above the unit is fed from the 24 volt supply from the PA rack and is an indication that the rack is on.

The mains switching unit is operated by a battery coming from the level 6 relay set to operate relay RLC. This in turn operates relay RLB which holds and also operates the mains relay RLA which extends the mains to the 13 amp socket feeding the PA rack. When the MO wire battery clears the relays continue to hold whilst the mains supply persists and keeps the PA rack operational.

There is also a time switch (not shown) in the mains circuit. This timer switches off the mains supply for one minute at 19:30 at the end of most working days. This releases the relays in the control unit and turns off the PA rack. The power to the rack can only be restored by dialling 6 or pressing the three hour button.

Please note in the diagram below that the contact in the timer is normally closed and thus allows mains through to the switching unit. The contact opens for one minute at 19:30 each day to turn the PA off for the night.

The final fall back should all else fail is to enter the exchange and move the PA 13 amp plug from the controlled socket to an ordinary 13 amp socket in the room. This has been necessary in the past when the 50 volt battery supplying the exchange failed.

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Page provided by John Bathgate This page was last updated on 9th May 2008