In our project, terminal blocks type used for DCS Analog I/O is "Knife disconnect type" while for DCS Digital I/O is "Disconnect type with blown fuse indicator".

I would like to know what is the reason behind that design.

Is it possible to use "Knife disconnect type" for Digital I/O? If yes, why they didn't prefer to use it since i guess it is much cheaper.

 

Fuses provide short circuit protection, which is useful for digital outputs. It is generally not considered necessary for digital inputs, analog inputs nor analog outputs.

The knife switches are useful for maintenance and testing.

 

The fused blocks are used for wire protection. May or may not be required by some authorities. Analog I/O is current limited internally. You can use the cheaper blocks for digital I/O but then you need to add fuse blocks. The fused terminals may be cheaper than the combination.

Regards
cww

 

Knife and fused terminals are very useful in commissioning as it can allow selective testing very quickly and easily. Knife terminals can also be useful in calibrating 4-20mA loops etc where the loop needs to be broken for checking etc.

 

Hi there,

Saying generally not necessary for digital input, analog output and input is very easy. I'd like to know why, kindly give me some technical explanation.

 

Analog inputs and outputs are typically 4-20 ma current sources, thus limited already to 20 ma. Even shorting to ground you would be unlikely to blow the fuse. Thermocouple temperature inputs are even lower current devices and RTD's are passive and have extremely low current passing through them.

Digital inputs are also low current, with the power supply current limited, so the fuse would provide little protection.