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Selecting Protection - Signal


Process control protection must:

1.    Provide adequate protection for all equipment. 
2.    Achieve a long working life. 
3.    Allow the signal to pass under normal operation. 
4.    Optimise the cost and size of the surge protection devices (SPDs).

Options for Surge Protection Devices

Two port SPDs are connected in series with the line. Almost all Novaris process control SPDs incorporate this configuration where a low let through voltage, (Up) is always required to adequately protect low level signals. 


Novaris process control SPDs contain a combination of voltage switching components comprising gas discharge tubes, series impedances and voltage limiting components comprising MOVs and suppressor diodes.

Selection of Surge Protection Devices

The selection of SPDs for process control requires more attention than the selection of power line SPDs to ensure the signal is not attenuated or lost through the SPD. 
Novaris manufactures process control SPDs for almost all applications and can design custom solutions for unique applications. 

1. Determine the signalling protocol and peak line voltage 
Table 1 on page 60 provides common signalling protocols and the appropriate Novaris SPD for each application. Even if the actual protocol is unknown the peak signal voltage must be determined. 

2. Select the clamping voltage 
The clamping voltage of the SPD must be greater than the peak signalling voltage. 
The following is a guide.





3. Determine the signal current 
Standard SL models are rated at IL = 350mA. For current up to IL = 6A use the SSP6A series. For higher current applications, consider using SFD surge filters. 

4. Select signal frequency / data rate 
Standard SL series will pass signals up to 250 kHz. For higher frequency / faster data rates consider the SL485 or SL-DH. 

5. Consider earth isolation 
The normal SL DIN rail base, designated -G, connects the protective earth to the DIN rail to provide a low impedance earth path. If the earth must be isolated, for example with instrument loops, use the -EC90 base.