Bradford White MII Series Service Manual page 12

Sequence of Operation:
600V Surface Mounted Thermostats (w/Contactors).
The system has two distinct circuits.
1. Power circuit - Line Voltage (600V)
2. Control Circuit - 120V
Line voltage is applied across terminals of fuse block
1 or a terminal block. Line voltage continues down and
connects to terminals L1, L2 & L3 of one or more
contactors.
A) Contactor is open (no call for heat), so there is no
voltage across terminal T1, T2 & T3
of contactor.
B) The contactor is controlled by the control circuit
consisting of the surface mounted thermostat,
ECO (hi limit) and contactor coil.
When the thermostat calls for heat, contacts close
2
inside the thermostat completing the circuit through
the ECO and energizing the contactor coil.
The energized contactor coil causes the contactor to
3
close, energizing the elements from terminals T1, T2
& T3 of the contactor.
When the temperature setting of the thermostat is
4
reached, the contacts in the thermostat open. This
interrupts current flow through the control circuit de-
energizing the contactor coil.
The de-energized contactor coil causes the contactor
5
to open, interrupting current flow through the
elements. The heater is now in stand-by waiting for
the next call for heat.
Notice in the wiring diagram at right that each row of
6
elements is equipped with a surface mounted
thermostat on the highest element in the row. These
thermostats are individually wired in series to the
contactor whose line voltage corresponds to the same
row of elements. Each thermostat and contactor
combination are wired to the high limit (ECO) device
in parallel. This allows for each row of elements to be
powered "ON" independent of the other element rows.
However, the high limit (ECO) is wired in series so
that voltage can be interrupted to the entire control
circuit if the ECO senses that the tank water has
become overheated.
12
SEQUENCE OF OPERATION
6 Element Configuration Shown
(9 Elements Possible)
Fuse Block
Transformer
60VA
Page 12
Line Voltage
Contactor
High Limit
Heating Elements
59T
59T