Motor Protection

CT7N Thermal Overload Relays


Overload Protection

CT7N Bimetallic Thermal Overload Relays

Ensure reliable protection with current-limiting calibration & ambient temperature compensation

Choose CT7N overloads in DC applications and when monitoring Variable Frequency Drives

Sprecher + Schuh has always paid particular attention to the subject of motor protection. This concern is reflected in our CT7N line of thermal overload relays that include many standard features not available with eutectic alloy overload blocks and heater elements of the past. And, CT7N overload relays are designed to mount directly to the load side of Sprecher + Schuh CA7 Contactors.

Consistent and reliable protection

The consistent high quality of Sprecher + Schuh thermal overload relays is ensured by a complex current calibration procedure performed after each unit is at full operating temperature. Calibration is performed at the largest and smallest current the overload can handle. The accurate time/current characteristic curve obtained in this manner guarantees reliable motor protection every time.

Differential Tripping DiagramSuperior Class 10 characteristics

Today's T-Frame motors have less copper and iron that the old U-Frame motors that were popular when traditional Class 20 overload relays were designed. For this reason, faster Class 10 overloads like the CT7N Series have been recognized by many motor manufacturers as the ideal type to assure optimum protection of "T" frame motors.

Protection from single phase conditions

A unique feature not found in traditional thermal overload relays provides accelerated tripping under single phase conditions. This is accomplished with a special "differential tripping" mechanism built into CT7N (see illustration).

Ambient temperature compensation

All Sprecher + Schuh thermal overload relays are temperature compensated. An additional bimetallic ambient compensation strip, built into the conductor- bimetal transmission path, ensures that the tripping characteristics of the relay remain constant over an ambient temperature range of -20C to +60C.

Single phase applications

CT7N Series thermal overload relays can be applied for protection of single phase AC motors. The relays have the same characteristics as shown for three phase operation. To maintain these characteristics, each element of the overload relay must carry the motor current as shown in the schematic in the wiring diagrams library.

Other standard features

CT7N thermal overload relays feature a fail-safe "trip-free" design that prevents the device from being held closed during an overload. In addition, a selectable reset button permits either manual or automatic reset modes. A separate NO signal contact is also provided on CT7N overloads which is isolated from the NC trip contact. This permits the use of a trip signal voltage different than that of the control voltage.

International Standards and Approvals

Sprecher + Schuh CT thermal overload relays are UL-listed and CSA-approved. They also carry the CE mark.