Do analog 4-20mA inputs need protection varistors if they are fused?

I have some 4-20mA inputs which I'm protecting from overcurrents with 32mA fast action fuses, per recommendation from the manufacturer's data sheet.

Do I also need to add a varistor (MOV/VDR) across each for high-speed transient suppression, or should the fuses be sufficient?

Thanks!

I would wire as per the manufactures specification. If you experience transient surges in the field then add protection to the system.

Regards,

The answer depends on a lot of external factors like the type of sensors being used and the ambient conditions, but adding a suitable MOV certainly can't hurt. They should at least be able limit the incoming voltage spike long enough to blow the fuse.

I have never used varistors/MOV for protection of analog signals. Best method is to avoid running with high voltages and use shielded cable. Ensure cable is shielded at one end only (at the source) to avoid ground loops.

Here's the issue.

2 wire loop powered devices have a cable where one of the two wires carries 24Vdc(+). Should that cable, which sometimes runs hundreds of yards/meters be crimped or damaged, the 24(+) can short to the other wire which connects to the AI(+), as shown in the diagram.


Every Analog input has a dropping resistor to create an IR drop, a voltage, that the input uses for a measurement. It is a precision resistor that is typically not designed to carry much more than 25-30mA.

The transmitter typically drops 10-14Vdc and then limits the loop current by regulating the remaining loop voltage that the AI sees.

A short, an abnormal fault that puts 24Vdc to the AI(+) means that the AI resistor is now the loop current limiting resistor and it will have to dissipate the all the loop current. If it is a 250 ohm resistor (common, but not universal, many are 50 or 100 ohms), it would have to dissipate 2.3 watts, which is likely to burn the resistor out, because the resistor is probably 1/10W or maybe 1/4W.

This whole thread made me wonder why anyone would fuse analog inputs (I've never seen this before). Is fusing analog inputs a common practice in some fields? I have seen very granular fusing of the 24VDC for sensors to prevent the sensor cable shorting and turning into a little toaster coil, but never fusing on individual inputs.

The datasheet for this Click module does say there is logic side isolation up to 1500VAC for 1 second but it doesn't give a continuous rating or say it fails open. The overload is +/-44ma which isn't too bad, but it doesn't give a voltage overload (every analog card I've ever used is at least up to +/-25VDC to protect from accidentally putting 24VDC on it).

The IO I'm used to is fine up to +/-30V and +/-50ma with isolation between the inputs and the IO bus of 500Veff. Not much better than this Click. They're about twice as expensive as the Click, but still a lot less expensive than the T&M to fuse the inputs, so even if you did blow a card, you can fix the issue and pop in a replacement and still come out on top.

CapinWinky said:

This whole thread made me wonder why anyone would fuse analog inputs (I've never seen this before). Is fusing analog inputs a common practice in some fields? I have seen very granular fusing of the 24VDC for sensors to prevent the sensor cable shorting and turning into a little toaster coil, but never fusing on individual inputs.

Click to expand...

It all depends on what hardware you are using. I've used both that requires fusing/protection for the input and more modern I/O-cards with built in short circuit protection.

The power supply still needs to be fused though

1. Cpt: I've seen AI fusing on the occasional DCS (some old Foxboro's that ran 10-50mA were fused) and an occasional PLC.

I never hear about the cable accidents that might take out an AI, but the 3 burned input resistors I personally have witnessed were caused by

1) the subterranean vault where the pressure transmitter was located flooded; the dirty water was conductive enough to conduct enough current to burn out the AI input located high and dry above the flooding.

2) incompetent technicians who shorted the (+) and (-) terminals at the transmitter for reasons unknown, even to them.

I'm sure there are cable faults that I don't hear about, but I'm with you, why bear the cost of 'fuse' insurance for so little prospective return? Most of the world seems to live without it. The manufacturer's cover their butts by recommending the practice.

2. MOV protection is used where lightning protection is needed or maybe EMP protection for military service. Commercial surge protectors use MOV's. I see surge protection used where experience shows it's needed - water/waste plants with long, outdoor cabling runs. Occasionally in a steel mill for outdoor cabling, but rarely inside an all metal building.