Depends on how the sensor operates but a voltage drop could easily cause a false alarm.

What specific sensor is it?

Keith Cress
kcress - http://www.flaminsystems.com

I would imagine that the alarm output for a safety device like a gas detector is designed to fail "safe", meaning to fail in the active alarm state. A low voltage condition can trigger the electronics to either sense a failure and declare a failsafe mode - active alarm - or to fail altogether in a failsafe mode.

I've seen gas monitoring panels with UPS power supplies whose real purpose is not trying to maintain gas detection during an extended power failure as a much it is to provide continuous power through short, intermittent electrical power drop-outs, the kind that make light bulbs 'blink'. Those short drop-outs can send instrumentation into a power cycle during which the output alarm relay might be in an indeterminate state or held in a fault state until the electronics wake up.

What kind of output signal of gas detector?

It's 4-20 ma output signal. How to solve the issue of this voltage drop please. Do we simply increase cable size or sth else?

Insufficient voltage in a 4-20mA loop means that the mA output is limited to the current that the available voltage can drive through the loop resistance, which means a lower then expected mA output level. Is the output a loop powered 4-20 powered by a external power supply or power from analog input?

Insufficient voltage to operate the sensor electronics can manifest in a variety of ways, particularly if the sensor electronics use a lot of current.

What sypmtoms and analysis lead you to believe that this is a voltage drop issue? The solution depends on what the problems are, not stabs in the dark.

Hi Danw2

Thanks for your answer. The detector is powered by an external power supply. The symptoms are that, detector is frequently generating nuisance high alarms. It's troubleshooted as voltage drop!

Apart from increasing cable size, anything else we can do , from power supply side for example?

A capacitor across the supply (at the alarm) might help with high frequency noise.

Hi Brian

Thanks . I dnt think it will be applicable in my case..

helpyou,
What size of cable? Does it have screen? Is it twisted? How many cores connected to detector? Is there junction box between detector and PLC?

Hi guys,

Was wrong about power connection. Detector is loop powered (not external power supply). Its a two core wire, and there's a junction box and termination panel between detector and plc.

Thank you

A 2 wire loop powered device has only two terminals, a (+) and a (-). An external DC power supply supplies power over those two wires. But 2-wire loop powered technology is limited to a maximum draw of 20mA under normal operation, 22-23mA when signaling a fault. Is your device 2, 3 or 4 wire?

Many detectors require more current to operate the sensor, and are either 3-wire or 4-wire devices, with terminals for DC power and separate terminals for signal out. Some have inrush of 1A and operate at roughly 0.5A. The manual for devices that need 'lots of current' typically state what the current/wattage requirements are for the device and the DC power supply has to be sized accordingly. If the power supply can't provide sufficient power to run the sensor properly, then any number of scenarios can occur depending on manufacturer/brand and their diagnostics and alarming functions. What current does your meter draw? What's the wattage rating on the power supply?

Is the copper wire cabling on the order a thousands of meters, hundreds of meters, or tens of meters? What size conductors?

Most gas concentration is looking a minimum LEL value, a high alarm above the LEL. If the PLC generates an alarm by testing the 4-20mA signal and the 4-20mA signal gets to the high alarm level, then the 4-20mA signal is not being inhibited by low voltage, a low voltage would prevent the 4-20mA signal from getting high enough to trigger an alarm.

If the DC power is intermittent, turning on and off, then some field devices might go to full scale 20mA (+) during initialization when power comes back on, triggering nuisance alarms.

HI danw2,

My device is 3 wire loop powered from the plc analog card. What would be the solution if power from the loop is not enough for the detector? Should be install an external power supply?

Thanks a lot for your answers ..really useful

What size of cable, mm2?
What is model of the detector and type of sensor head(infrared, catalytic or chemical)?
What gas is measured, flammable or toxic, H2S? If H2S, then it could be poisoned.

One more question, has this gas detector loop ever worked properly in the past, but is now a problem or has this loop been a problem since the day it was commissioned? Are trying to get it work correctly because it never has worked properly or are you fixing a situation that previously was not a problem?

>My device is 3 wire loop powered from the plc analog card. What would be the solution if power from the loop is not enough for the detector? Should be install an external power supply?

It depends the PLC AI/power supply specs, the gas detector's current requirements, and to some extent, field conditions.

Field conditions: could other devices using the PLC power supply be intermittenly loading the power supply causing this condition? If so, then an additional power supply is a solution.

MaratA brings up points that warrant consideration - has the sensor successfully been field calibrated recently to prove its functionality?