Merger External & internal power for Gas Detector

Hi all...

need solution for this case..I have signed 1 Ea Gas detector ( use External power +24Vdc) to DCS Yokogawa. before it, 4 ea Gas Detector already signed to that Analog input modules (using internal power),and 1 Ea (External power) also merged to same module, there are 4 Ea GD use internal power & 1 Ea GD using external power. all devices running normally,but..sometimes,I found while 1 Ea gas detector (use external power) loss power (+24Vdc),will caused 4 Ea Gas detector (internal power) will show Hi-Hi alarm with values 50 %LEL,whereas the actual not gas leakage present. and tried to get back online,all condition back to normal,and 4 Ea GD will show normal value.

So, why it could happen like that ? can some body explain to me?please..thanks.. :site::site:

I suspect what you've encountered is a ground loop formed in the fault condition.

It isn't exactly clear what is powered by what, but it sounds like you've mixed four wire devices (device's power supply supplies the power to run the device's analog output) and two wire devices (an external power supply is wired into the loop and supplies power for the analog output) on a single analog input board.

A classic manifestation of ground loop problem is a signal driven off scale, up or down depending on the polarity of the ground loop.

In your case, the ground loop drives the internally powered devices' analog output upscale, triggering the hi-hi alarms.
Grounding one side of a power supply's output used to power a loop powered transmitter can create ground loops.

Another solution is to use a loop isolator between the 4 wire devices and the analog input.

Or live with the current situation.

thanks danw for your answers..

I think the problem comes from ground loops. so, if I relocate to a new AI module (empty)for devices with external power (stand alone) ,whether this could happen again? thanks..

Geragai,

Moving the externally-powered devices to a separate Analog Input Module should prevent the ground-loop problem. It is great that you have a spare Input Module available!

thanks Lancie1 ,

I`ll do that..