M970-07/04
PAGE 12
The operating supply input can be 3 phase 110 V. per phase at 16 amps for the end zone
and 13.5 amps for the centre zone. This is fed through a 3 pole latching 20 A. per line
isolation relay to each of the 3 furnace heater zones via the series “regulator” (solid state
control relay). This allows the 5 kW. total load during maximum heating power to be
divided onto the 3 phase supply (1 heater zone per phase).
As shown, an alternative single phase supply at 110 V. 5kW.(42 amps) can be used by
connecting the input 3 phase terminals in parallel . This single phase supply is fed via the
same 3 pole latching relay to the 3 furnace heater zones.
A separate fused (3A) supply from the main terminals after the input supply circuit breaker
feeds a distribution bus-bar for the local control equipment (controllers, relays, indicators).
A furnace power ON button with green ON lamp operates the 3 pole latching 20 amp. per
line isolation relay RL2 to the 3 separate furnace zone heaters and solid state control
relays.
A separate furnace power OFF button with red indicator lamp switches off the 3 pole
isolation relay supply to the 3 zone heaters and solid state relays to isolate main power to
the furnace, while leaving the temperature controllers operating.
Over temperature alarm and “cutout”
An entirely independent TC on the centre zone and an over temperature controller on the
front panel can be set to provide an “alarm” in the event of the furnace temperature
exceeding a pre-set high limit. The alarm controller closes a set of output contacts which
operates a separate control relay RL1 to trip the main furnace 3 pole relay RL2 to isolate
power to the furnace, switching on the red front panel over-temperature indicator. This
protects the furnace if any fault causes permanent full power to be applied which would
cause over-heating of the furnace and “burn-out” of the windings
