Please pay attention at five parts when you try to select SPD for signal.

1.  Maximum operation voltage Uc

The maximum voltage of signal circuit is less than Uc, normally:

AI/AO/DI/DO -> 24V

RS485/RTD/TC -> 5V

RS232 -> 12V

2. Nominal Operating Current IL

The maximum current of the circuit is less than this value:

Since SPD for signal is installed in series，the operating current will go through SPD. If the current is bigger than IL, SPD will be damaged(resistor overheated and burns).

3. Number of Wires: 2,3 or 4

Thermocouple is general 2-wires, and thermal resistance is general 3-wires;

4. Intrinsic safety application or not

Ex type can be used in intrinsic safety application, Normal type can’t. And if it is intrinsic safety application, isolated barriers will also be needed.

5.  If is screwable SPD, also pay attention to the male thread .

 1/2"NPT, 3/4"NPT or M20×1.5

Regarding to the naming rules of CHENZHU signal SPD, for example, T-24-EX-L4.

CHENZHU has two types of SPD for signal: Ex type and Normal type.