Induced AC Voltage on DC fire alarm wires

[tom baker]

I had a question on how to reduce or elimante induced AC voltage on a outside municipal fire alarm system.
This is an old pull box system, that has been converted to digital. The system is a series type, 72 to 120 VDC, runs outside on poles under utility power lines, total run 15 miles. A small amount (7-15 VAC) is being induced and this is interfering with the digital system. Is there a way to filter out the AC component?

 

[hbiss]

Re: Induced AC Voltage on DC fire alarm wires

If you are measuring this voltage with a properly loaded meter then I would say it's accurate. This doesn't necessarily mean that it is an induced voltage however since we don't know how you are measuring the voltage and what the other end of the wire is connected to.

I would say that this is a question to ask the alarm equipment manufacturer anyway since this is probably not an unusual installation hence that voltage should not be causing a problem to begin with. I wouldn't assume this to be a problem on my own without talking to them first.

-Hal

 

[steve66]

Re: Induced AC Voltage on DC fire alarm wires

I assume you have a pair of wires running on the poles. I also assume that the two wires are part of a cable as opposed to separate wires like the utility runs.

A resistor across the lines on each end might reduce the AC component. But, it will also reduce the desired signal. With a little more info, it should be possible to calculate resistor sizes to give the best performance.

If this doesn't give satisfactory operation, one would have to use a more complex filter. The problem is to leave the DC, eliminate the 60Hz AC, and leave the digital signal. The digital signal should be modulated, so it may be at 1200 or 2400 Hz. Some design work would probably be required, and again, one would have to know more details.

Another thing to think about is grounding. Is one wire grounded, or are both lines isolated? This brings up common mode or differential mode noise (not my strong point, so I will leave it at that).

Steve

 

[steve66]

Re: Induced AC Voltage on DC fire alarm wires

hbiss brings up a point I also thought about after posting. Are you sure the 7-15VAC you are getting is 60 Hz? Or is it possible that it is part of the modulated digital signal?

Steve

 

[dereckbc]

Re: Induced AC Voltage on DC fire alarm wires

Siganl transformer with DC pass-through.

 

[tom baker]

Re: Induced AC Voltage on DC fire alarm wires

Thanks Dereck I was looking for your answer.
This is a single conductor series system. The person who asked the question was discussing today with a fire alarm tech, who used to work on these systems, the recommendation was to contact the mfg.
My last two posts were questions from a grounding class. I always tell my students the Mike Holt code forum is a #1 resource and the anwers prove it.

 

[dereckbc]

Re: Induced AC Voltage on DC fire alarm wires

Tom, I do not know what type of signal is being transmitted on the line, but a simple capacitor connected from line-to-ground would remove the AC component, or a series choke in the line. If you had some sort of AC signal being transmitted, then you you would have to use either a signal transformer or high-pass filter.