Harmonics

[jazzman]

Hi all,
Harmonic interference from fluorescent lights is a subject that I don't know much about but its time I learned. Here is my problem:
I have a nursing home customer that has some patients that are not permitted to go outside by themselves. The exit door is equipped with an alarm system that sounds when a restricted patient approaches the exit and is within 4 or 5 feet of the door. There is a device that care givers attach to them that sets off the alarm. This allows anyone else to enter or exit without setting off the alarm. This alarm system was working fine for years until we retrofitted the existing T12 ceiling fixtures with T 8 ballasts and lamps. The alarm system now allows the restricted patients to get within one foot of the door before alarming which reduces the response time for any caregivers to intercept the restricted patient. The alarm system is adjustable but one foot is the best we could do.
Here is the real puzzler: Since the alarm system is fed from a generator back up panel, we shut off the breaker that fed about one of every four ceiling lights from the same panel. The exit door alarm system operated the way it did before the retrofit. It was perfect. None of the other ceiling lights that were not on generator back up and were still turned on had any effect on the exit doors alarm system. I determined that the gen back up ceiling lights and alarm system were being fed from the same phase so I moved the lights to a different phase. No difference. So we tried replacing the ballasts in the gen back up fixtures with high efficiency ballasts. It actually made the problem worse.
The maintenance director is anxious to get things back to normal. So I am thinking about installing the low voltage wire for the door alarm system in EMT since it now travels through the ceiling without any protection. I welcome any thoughts on this matter. Thanks in advance.

 

[__dan]

Hi all,
So I am thinking about installing the low voltage wire for the door alarm system in EMT since it now travels through the ceiling without any protection. I welcome any thoughts on this matter. Thanks in advance.

Electronic ballasts do give off some RFI, but I would not know if the RFI is travelling through the air or through the wiring.

It seems in your application the door proximity sensor is reading some type of RFID tag on the customer. The sensor could be picking up RFI noise through the air, between the tag and the sensor, or through the sensor wiring, between the sensor and the control input. Noise picked up though the sensor wiring would be the easiest to fix, with some simple type of wiring signal conditioning circuit added to the sensor output, or rerunning the sensor circuit to reduce induced noise on the sensor circuit. I would consult with door control or sensor manufacturer for their recommendation, but shielding of the sensor wiring as you suggest would be one of the trial solutions.

If the RFI is coming through the air and interfering directly with the RFID tag reading, that problem could be harder to fix. Again, if it were me, i would consult with the door control or sensor manufacturer about the problem. Once the cause is identified, how the RFI is being picked up and interfering with the sensor, the sensor manufacturer may be able to recommend a resolution.

You would have to assume the door control manufacturer is deploying these systems many times everywhere, at sites that all have electronic ballast lighting systems. This may be a problem they have seen before and can make a recommendation.

 

[junkhound]

Do you know the type of alarm system?
RF or magnetic or resonant magnetic?
e.g if the patient wears a flat chip as on a badge, it is likely an RF system, if they wear a small bar like device as seen on library books and some store items, it is magnetic, if a flat bar like device also seen in stores and on some books, likely resonant magnetic. .

If the system is a magnetic sensor it is possible the harmonics from the ballasts are partially saturating the detector circuit in the door sensor pickup.
All electronic ballasts have a full wave rectifier bridge that drives the DC link which powers the high voltage inverter. These rectifiers generate harmonics (even harmonics driven from single phase inputs). Ironically, high efficiency ballasts usually do not have PFC circuits (power factor correcting) that reduce the harmonics conducted out onto the power wires. Shutting off some lights as you describe reduces the total harmonic levels, so the sensors work better as you observed. See below for more probable cause though.

If the RF or resonant magnetic type sensor, conduit may help, doubt it will help on harmonics on a magnetic detector system.

However, from your descriptions, I'm 'guessing' your facility has the resonant magnetic type system. That system sends a 60 kHz or so signal out from the detector that causes the patient device to resonant, that resonance is picked up by a detector that shows the patient proximity. Unfortunately, the inverters and PFC circuits in electronic ballasts generate a LOT of 30 to 200 or so kHz EMI resulting from the inverter FETs or IGBTs switching at near that frequency, both radiated and conducted - this interferes with the detectors. Metal Conduit should help with a problem on this type system. A further assumption is that the wiring to the lights is already in metal conduit? If not, that would help also, but only if ring compression type fittings are used (set screw fittings do not work well using emt conduit for shielding)

Detailed investigation of this type interference in critical systems usually involves looking for and filtering the source. 'Looking' requires the use of a spectrum analyzer or at least a good scope.

Good luck. -- as Dan said, consult with the door control or sensor manufacturer

 

[jazzman]

Thank you both for your help. I will be installing EMT with compression connectors on Monday. If you check this post on Monday evening, I will post the results. Thanks again.

 

[ritelec]

I will be installing EMT with compression connectors on Monday.

Maybe help the economy out............
Can the facility hire someone to sit at each door with a baseball bat?

No more need for the ankle bracelets...........puts a person or two to work?????



Playing !!!

Good luck with it.

 

[junkhound]

If you check this post on Monday evening, I will post the results

So?? what were results? Above quote was from more than a week ago.