AC Interference on Digital Input Circuitry


Thread Starter


hello every body

a strange problem happens when Battery DC supply get mixed with AC component (due to charger earth fault) in a power transmission substation.

some devices (Bay control units, tele-protection equipments,...) malfunctioning and their Digital Inputs get triggered even though, there is no actual input from field.

I wonder, how is that happens? bad input circuitry design? what is the theory of DI interface circuits used in todays protection and control equipments?

any discussion or comments will be highly appreciated.

thank you.

PLC Engineer

For a DI, any spike greater than 12V is considered as an input.
Although the signal input might be designed to source or sink wiring, an AC component greater than 12 V (Depends on the error reading capacity. Ideally should be 12 V but can pick up in much lower voltages) if affects the input or common ground (If frequency less than the max acceptable freq pulse), will be considered as a DC signal, triggering the function.

The input is considered Di cause it can consider only two states Logic 1 for 12v ideal and Logic 0 for 0V ideal. This can be reversed in sink wiring.

Whereas AI can identify multiple stages.

This has been the general condition with PLCs and DCSs.

Protection is provided by Switching power supplies. Isolate DC ground from AC ground. (Use individual earthing cables).

Further you can use filters like inductive coils at the DC power supply. Use voltage regulators running with switching power principle.

Hope this answers your concern .
@PLC Engineer:

Thank you for your valuable response.

my concern is about spikes on DC power supply of device and its effect on its DI circuits. since in my case - there is no spikes/chattering applied to the DI and its completely isolated from field.

thank you.

PLC Engineer

Dear Elabbassy,

Any analogue component in your DC supply outlet will affect the DI. It is because, the DC is directly used as input, controlled through a signal isolators such as opto-couplers or Relays.
Meaning, you are using the same DC source to supply input signals to the controller by switching, not direct signal from the field (xcept when you use modbus/profibus/RS232 etc).

You might be using isolators/couplers for this purpose generally.
This is to prevent any distortion of signal from field.
Also the voltage range/ Ampere range of the DI might be different from your field devices'.

If the signal is fed directly, the grounds and power terminals will be shorted to ensure same operating voltage range, as the 0V that your DC source provides and 0V of the DC source of the field instrument needn't be same. There are chances of potential difference.

As the issue is the distortion in power source, isolators or couplers cannot help. Only filters can.

Inductive coils in series to a dc source acts as impedance for analogue components. A capacitor in parallel, acts as a short for analogue components, shorting the source and ground for analogue, but acts as Resistance for DC components, preventing short.

These are the commonly used simple filters for a DC source.

Hope the solution helps.