The 59T and 66T series controls are designed to meet the high electrical capacity requirements of electric water
heaters. Both use a temperature sensitive bimetal disc to deliver snap-action to the contacts. The speed and force of
contact separation provides long-dependable control life at high electrical loads.
Features and Benefi ts
The 59T and 66T series features include:
? The 59T mounting tabs snap into the customer’s bracket to mount
the thermostat on the surface of the tank
? The M2 trip free manual reset 66T limit control is available with
non-adjustable calibrations from 160° to 235°F (71° to 113°C)
? The 59T has an adjustable range of approximately 60°F (33°C)
The lowest adjustable limit is 90°F (32°C) and the highest adjustable
limit is 200°F (93°C)
? Controls are 100% operation checked
59T, 66T Series
Electric Water Heater ControlsBimetal Disc Thermostat
Therm-O-Disc
81
Switch Actions
The 59T is available in two switch actions:
Automatic reset SPST – The switch opens the normally closed contacts on
temperature rise. The contacts automatically return to the closed position
when temperatures return to the reset point.
Automatic reset SPDT – This operation is the same as the SPST with the
addition of an auxiliary set of contacts, which make circuit upon opening of
the normally closed contacts and breaks this circuit upon automatic reset.
The 66T is a manual reset DPST – The switch opens two sets of normally
closed contacts on temperature rise to provide full power disconnect of
both conductors. The contacts will reset to the closed position when the reset button is depressed after the control
has cooled to 90°F. Once opened, until the reset button is pressed, the contacts will not automatically reset at control
temperatures above 32°F (0°C).
The construction is classifi ed as “M2 Trip Free” by the approval agencies. This design holds the contacts open in
the event the reset button is held in the depressed position in an attempt to defeat the manual reset function of
the thermostat.
Typical Application
The thermostats and wiring diagram (shown on next page) are typical of a residential electrical storage water heater
installation using two heating elements for non-simultaneous operation. The upper electrical control is a combination
59T SPDT regulating thermostat and a 66T DPST manual reset limiting thermostat. The lower electrical control is a
59T SPST thermostat. All the thermostats sense the surface temperature of the water heater tank.
The 66T function is to provide full power disconnect (through both power conductors) in the event of an overheat
condition. The 59T SPDT normally closed contacts route power to the upper heating element. When the 59T SPDT
control reaches its set temperature, its switch action breaks power to the upper heating element and switches power
to the lower heating element through the normally closed contacts of the 59T SPST. When its set point is satisfi ed,
the switch action breaks power to the lower heating element.
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