ECN Forum Forums General Topics of the Trade General Discussion Area radio interference filter

I recently wired some operating rooms in a hospital. Each room has an isolation transformer fed panel that is monitored for any ground faults (the milliamps are displayed on two analog and one digital meter). If the milliamps climb too high, the system goes into alarm. In this case, once juice was applied, the system did indeed go into alarm mode. After doing some investigating, it was found that the radio interference filters inside the fluorescent lights were causing the problem. These filters are tied in series with the fixture ballast line wires. With the filter bypassed and only the ballast hooked up, everything was fine and the monitor read no loss of current.

We have not been able to get a hold of the electrical engineer on this project yet so I thought I'd do some investigating of my own (that’s where my fellow forum users come in). Do these radio interference filters send a small amount of current to ground while performing their duties? If so, they obviously should not have been spec'd for this application. Is there a higher-end filter that would not cause the problems I'm experiencing? I could probably isolate the metal case of the filter from the fluorescent fixture but that may negate or decrease the effects of the filter and probably would not be safe or UL approved. There would be no place for heat or current to dissipate and the fixture would be modified from its UL approved set-up.

I'll probably have an answer back from the engineer by early next week but my curiosity is building!

these filters have capacitors from hot and neutral to ground.

I can't tell you if your ground fault current was normal capacitor loss or a faulty cap's leakage.

I think using filters would be better because the starting fluo fixtures send quite big radio strikes back into the line. Perhaps you can find some extra low loss filters or sth.

[This message has been edited by :andy: (edited 02-28-2004).]

I don't think I have a faulty filter. 2 of the 3 OR's are already fired up and they both go into alarm. As each of the 10+ fixture filters are unhooked from the circuit, the amount of lost current is reduced. Thus, all filters seem to be performing the same. It seems doubtful that all units could be bad.

Thanks for your response. At least I have a little better understanding of what may be going on.

The RFI Filters are working correctly, and acting normally, to filter out what is figured by the filter to be interference.
Those parameters are the design points.

These AC Line Filters are most likely built as a typical PI filter with a common mode Inductor on both lines - instead of a single Inductor on one line.

At the line side of the Inductor(s), two Capacitors will be connected in series with each other, but in parallel to the Inductor.
These are connected to the AC two wire circuit at the input.
At the center of the series connected Capacitors, there is a tapped lead - which goes to the EGC (tap driven to ground). That's where the leakage current is coming from (or going to).

On the Load side of the Inductor(s), there is a single Capacitor in parallel. It connects directly to the AC two wire circuit, but has no "Ground Tap" like the input section does.

Some more elaborate Filters may have several "PI" stages - therefore may have more than one "Ground Tap Point".

These Filters will be low pass filters. They probably are designed for passing Frequencies upto 400HZ, and "Dumping" (rejecting) Frequencies >400HZ.
The higher Frequencies will be dumped to the Equipment Ground/Equipment Grounding Conductor, via the center tap on the input filter section.

This may sound funny, but an effective fix for this scenario could be done by using an Isolated Equipment Grounding Conductor that connects only to the filters' EGC terminal, then terminates in a location which can be detected by the leakage monitor, or possibly passes the monitor.

Other than this, the filters might need to be exchanged for more simpler types which do not dump to the grounding conductor.

I can think of 3 things being filtered out to ground, by these filters -
<OL TYPE=1>

[*]Harmonics,

[*]Carrier Current signals,

[*]Actual Radio Frequency Interferences.
</OL>

Curious to hear what the Designing EE has to say.

Scott35

triple,
Are you sure that these are RFI capacitors?.
It sounds to me like this is a Power-Factor Correction Capacitor in a Lead-Lag fitting.

But a power factor correction cap should not loose anything to GND.
As the ones in the filter package do, as said. But no idea how to fix this case....

Trumpy, the units in question are labeled "radio interference filter".


Scott35, these filters do not have individual ground leads or terminals. They are grounded just like the ballast itself. The housing is metal and is bolted to the metal fixture housing. Therefore, the filter would need to be insulated from the fixture. Grounding the filter ANYWHERE will cause the system to go into alarm since a leakage will be detected (the monitor reads the neutral and hot and checks for an imbalance just like a GFCI breaker).

Hopefully, I will hear from the EE tomorrow. Thanks for the responses.

Here is a simple fix, these fixtures are outside of the "Patient Vicinity" and can be removed from the Isolation System/s.

The 5 ma alarm level is becoming harder to maintain with more and more electronics being brought into O.R's.

A common problem is Hospitals not buying specific HG equipment and the leakages that are normal on this non HG equipment. (There are even HG TV's)


Any filter that has any reference to ground will cause the Lim's to alarm.

We have a minimum of 200,000 ohms at 120v >0006A at an idle state, which is the maximum allowed to commission the room into service, and a 24,000 ohm at 120v = 005A alarm level.

Capacitive coupling in itself do to poor insulation, (you should use the best xhhw available) or length of accumulative condutors will cause problems before any loads are introduced to the system.

The bottom line is the filters will have to go or the flourescents will have to be put on a non iso system as I said before.

Roger

[This message has been edited by Roger (edited 03-01-2004).]