A.O. Smith DRE 52 Installation And User Instructioins page 6

3 relays (3) and a control current
transformer (5) are fed from the main
connection block (1). The main circuit
is protected by 30 Amp T fuses.
The transformer (400V to 24V) is
primarily secured with T0,2 A (4). A boil
dry protection mechanism (in the form of
a fl oat switch) (7) is included in the 24V
current control circuit. (The fl oat switch
is indicated diagrammatically. See boil
dry protection further on for more precise
explanation) The control current circuit is
closed if there is water in the device (fl oat
switch). In this case an auxiliary relay (6)
is used which in turn switches the relay
(3). The relays determine whether the
electrical elements are fed.
The elements are fed via the relay (3).
The wire goes to the maximum thermostat
(8) via the relays (2). The maximum
thermostat has a reset button. The output
from the maximum thermostat goes to the
regulating thermostat (9). The element
(10) is fed from the regulating thermostat.
6
During normal operation the regulating
thermostat (9) switches the element (10)
on and off. If a temperature of above
93°C occurs on an element the maximum
thermostat (8) locks the element. In order
to put the element back into operation the
reset button of the maximum thermostat
must be pressed in. The temperature
must have dropped by approximately
20°C before the maximum thermostat can
be reset.
If the temperature of the water above in
the tank exceeds 98°C for any reason,
the combined temperature and pressure
safety device (T&P valve) will open.
The safety control circuit will be interrupted
if the water level drops to below the fl oat
switch (7) or if the device is switched on
while empty.
The device is then switched off completely.
This is to prevent the elements coming
into operation while there is insuffi cient
water present in the device (boil dry
protection).
The safety fuses, 30 Amp T for 600 V can
be ordered retrospectively from A.O. Smith.