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The following is the core difference and selection guide of the bimetallic KSD301, KSD9700 normally closed (NC) and normally open (NO) temperature switches, and a comprehensive comparison of technical specifications and application scenarios:
I. Working principle and state definition
| Characteristics | Normally closed (NC) | Normally open (NO) |
| Initial state | Contacts closed at normal temperature (circuit connected) | Contacts disconnected at normal temperature (circuit disconnected) |
| Trigger action | Temperature ≥ set value → contacts disconnected (circuit disconnected) | Temperature ≥ set value → contacts closed (circuit connected) |
| Reset mechanism | Temperature drops to reset value → contacts automatically closed | Temperature drops to reset value → contacts automatically disconnected |
| Symbols | NC （Normally Closed） | NO （Normally Open） |
Core application logic:
NC type: “high temperature power off” (such as preventing the motor from overheating and burning);
NO type: “high temperature power on” (such as over-temperature starting the cooling fan).
 Normally Closed and Normally Open Temperature Switches |
 Normally open KSD9700 temperature control switch |
 Normally closed KSD9700 temperature control switch |
II. Typical application scenarios
KSD301 Temperature Switch Parameters
| Type | Applicable equipment | Protection purpose |
| NC type | – Motor overheat protection (washing machine, air conditioner compressor) – Electric water heater anti-dry burning – Transformer overload protection |
When the temperature exceeds the standard, the power supply is forced to be cut off to avoid equipment damage |
| NO type | – Cooling fan control (computer CPU, cabinet) – Heater overtemperature alarm – Greenhouse heating system |
Activate auxiliary equipment (such as fans/alarms) when the temperature exceeds the standard |
III. Key considerations for selection
Safety priority
NC type must be selected for forced power off scenarios (such as fire prevention, electric shock prevention) to avoid accidents caused by continuous power supply at high temperatures.
Circuit design matching
NC type needs to be connected in series in the main circuit live wire, and NO type is usually connected in parallel to control auxiliary equipment.
Identification
“B” or “NC” marked on the shell indicates normally closed type, “K” or “NO” indicates normally open type (such as KSD301-85B indicates normally closed type at 85℃).
Test verification
Use a multimeter to detect: NC type is connected at normal temperature, and NO type is disconnected at normal temperature.
Normally open KSD301,KSD302 disc switch
IV. Avoiding operational errors
mermaid Copy Code
graph TB
A[Equipment requirements] –> B{Does it need to be powered off at high temperature? }
B –>|Yes| C[Select NC type]
B –>|No| D{Does it need to be started at high temperature? }
D –>|Yes| E[Select NO type]
D –>|No| F[Re-evaluate the design]
▶️ Summary: NC type is the “emergency brake” for safety protection, and NO type is the “temperature trigger” for intelligent control. The action logics of the two are complementary. The selection must strictly match the circuit function requirements (comprehensive).
Normally closed KSD301,KSD302 temperature limit switch