Audio (and radio) interference from fire alarm system

[MarkL]

Hi,

We have a Cooper Fire alarm system installed at a theatre where I sound tech. It has a Cooper Eaton CF3000 control panel. The fire alarm system is generating interference which affects magnetic pickups on electric and some acoustic guitars, and can also be picked up in and around the building on a Long Wave analogue radio. The interference appears to be coming from the fire alarm's addressable loop (the cable loop which connects the control panel to all the senders and sounders), and is being induced onto the mains cabling in the building.

Please see here for a more detailed description, https://drive.google.com/file/d/1ZFgNcuGsEdHObmYaZEV3sbaW28PvohaY/view?usp=drive_link , and please see here for various supporting files (e.g. recordings, videos, etc.) https://drive.google.com/drive/folders/1dzl8XW9ZuM1KOZ4E_guiQVqfBZFSpJG8?usp=drive_link .

Thank you in advance for your help!

Edited November 20, 2023 by MarkL

[DrV]

Employing the "someone else's problem" strategy, I would suggest that the manufacturer of the alarm system is responsible for this. They do not appear to be complying with the EMC directive which requires electronic equipment not to interfere with other systems.

Dave 

Edit: trying to be more helpful, I think you might be on the right track with the resistor but you would probably need some capacitance as well to for a low pass filter. However, if you modify the system and then the building burns down you might be on shaky ground if the manufacturer hasn't approved such a mod.

Edited November 20, 2023 by DrV

[DrV]

I've just realised you said you put the resistor in the screen connection, not in the loop connections. If you were going to add resistors to limit the slew rate they would need to be in the loop connections. And, as they also supply the power to the sensors they would need to be low ohms values. Probably less than 10R. This, combined with the capacitance in the wiring would probably be sufficient to lower the slew rate enough. The signalling rate seems to be about 1kBaud so fast edges should not be required.

But I still think you need to talk to Eaton about it. They are a reputable company and (unsurprisingly) the manual specifically states that they do comply with the EMC regs. They must have encountered this kind of problem before.

[Bryson]

It appears this system is essentially a giant induction loop but instead of passing audio, it sends (and receives?) signal to fire sensors.  Essentially, it's supposed to broadcast RF, although it should be broadcasting at a much higher frequency than you're picking up - it looks like designed frequency is 868MHz for that system - so it appears you're getting some kind of unintentional signal, maybe?  Your guitar pickups shouldn't be picking up anything beyond the Khz range really.

[DrV]

23 minutes ago, Bryson said:

  Essentially, it's supposed to broadcast RF

That's an intriguing idea. Where do you get that from Matt? The idea that they would try to use RF reliably over such unpredictable wiring as is used for a fire alarm (FP200 etc) seems a bit unlikely but I'm open to being educated!

[Paul TC]

20 minutes ago, Bryson said:

 - it looks like designed frequency is 868MHz for that system - so it appears you're getting some kind of unintentional signal, maybe?  Your guitar pickups shouldn't be picking up anything beyond the Khz range really.

One possibility is that you are picking up an intermodulation frequency from the addition / subtraction of two signals that are close in frequency giving an output in the KHz range.  Have you any other short range wireless devices in use as 868MHz is used for these.  Link to intermod webpage

[Bryson]

1 hour ago, DrV said:

That's an intriguing idea. Where do you get that from Matt? The idea that they would try to use RF reliably over such unpredictable wiring as is used for a fire alarm (FP200 etc) seems a bit unlikely but I'm open to being educated!

I just read the product info.  The sensors and so on are connected wirelessly, via a huge induction-loop style setup.  (It permits a loop of up to 2Km in length!)

[Robin D]

Don't Eaton also own Zero88? If so, it may be one that @jonhole may be able to mediate on.

[DrV]

2 hours ago, Bryson said:

I just read the product info.  The sensors and so on are connected wirelessly, via a huge induction-loop style setup.  (It permits a loop of up to 2Km in length!)

Thanks. Must have missed that - I only saw the wired version.

Edit: the OP did say it's a CF3000. Have now searched the installation manual and it doesn't mention RF, wireless or MHz anywhere. Are you sure you were looking at that model?

Edited November 20, 2023 by DrV

[richardash1981]

I presume you have the manual for the panel (https://www.eaton.com/content/dam/eaton/products/safety-security-emergency-communications/fire/addressable/cf3000-panel/en/eaton-fire-addressable-control-panel-cf3000-manual-25-14682-en.pdf).

Page 46 notes the admissible cable types for the system, basically all of them are two conductors within an outer metallic tube (screen). The rest of the cable spec is to do with fire protection, not EMC. There are then various requirements about not using multicore cable, not mixing loop out and return etc. Are you confident that these have been followed in the installation?

I'm inclined to agree with Dave that this isn't an RF system, it is basically a serial system with superimposed power (so that the devices don't need independent power), at a reasonably low baud rate. The question in my mind is not then "how to we stop this causing interference" but "what wiring defect is causing the system to radiate in unexpected ways".
My immediate thought is whether the outer of the cable loop(s) is grounded wrong. Page 48 is fairly clear that the whole loop should be floating except for a single point of connection to earth at the panel, and I'm wondering if this isn't the case (fairly easily done) and is the cause of the problem? Diverting current out of the fire alarm loop into the building mains earth wiring could easily (unintentionally) create the kind of giant loop antenna which others have described!

[DrV]

36 minutes ago, richardash1981 said:

Page 48 is fairly clear that the whole loop should be floating except for a single point of connection to earth at the panel

It actually says "Both the loop start and the loop end 
must be connected to the appropriate earthing points". As there are apparently no terminals provided for this that must mean 'through the cable glands'. The connection diagrams on pages 110 and 111 etc confirm this. But yes, the shield must not be grounded anywhere else and it's very specific about that.

Edited November 20, 2023 by DrV

[Andrew C]

If it is a loop, then both ends should be at the same place?  No?

[DrV]

25 minutes ago, Andrew C said:

If it is a loop, then both ends should be at the same place?  No?

Yes, as is common with alarm systems.

[MarkL]

First off, thank you very much for your analysis and responses so far. Dealing with the points raised:

- DrV: We temporarily replaced the connection between loop shield and electrical earth with a 150R resistor purely in order to see if there was any unexpected interaction between the addressable loop +- and the shield, bar the expected capacitive coupling. While the value was too high to keep the alarm panel happy, the 'scope trace of the loop shield does appear to show pure capacitive coupling from + and -, and appear to rule out any connection to + or - anywhere within the loop. The addressable loop may be driven by any single device, one at a time, in the loop. The particular sound of the interference is likely due to the pattern of the alarm panel stepping through each device checking it's ok, and each device responding. The fact that any device may transmit on the loop, in combination with the requirement that a single loop problem (e.g. cable short/break/on-fire) shouldn't stop the panel communicating with all devices on the loop means that the + and - conductors are not continuous on the loop - each device deliberately intercepts them. So, inserting a low value resistor in loop + and - in the panel would unfortunately only affect its transmitting to the first device in each direction, plus them transmitting back. Agree re talking with Cooper Eaton - we're about to do that.

- Bryson: All the fire devices (e.g. senders and sounders) in our system are connected to the control panel via a single wired addressable loop. None of them is wireless. There is a single short-ish spur off the loop, itself connected via a small Cooper box designed for that purpose. I don't think it's meant to broadcast RF. Re the guitar pickups not picking up much above a few kHz, I suspect they're picking up RF, then the downstream amplification (presumably with imperfect HF cut filtering at well above the audio range) is intermodulation-distorting that down into the audible range where we hear it.

- Paul TC & Bryson: The interference is definitely from the fire alarm system addressable loop - we've proved this. That said, good point re intermodulation distortion - I think that's the effect by which we hear the resulting interference picked up by by magnetic pickups through the sound system.

- richardash1981: I've read that part of the manual and, as far as I could tell, these cable requirements have been followed. You wrote "what wiring defect is causing the system to radiate in unexpected ways": Yes... Measurements on the loop shield show it to be continuous (once we'd fixed a single break) and with a single connection to electrical earth in the alarm control panel. The link to the detail shows a 'scope trace of the shield when this connection is broken and replaced with a 150R resistor. The trace looks, to me, like pure capacitive coupling. (The mains wiring does act as an aerial capable of picking up RF - see the spikes for BBC R4 LW (200kHz) plus Radio 5 on MW on the ClubFreq2MHz_2.PNG pic in the details link above. Suspect this is normal behaviour, though.)

DrV & Andrew: As far as we can tell from measurements, the shields at both ends of the loop are joined and connected to electrical earth at a single point in the fire alarm control panel, and the shield is not connected anywhere else.
 

Replies welcome and I'll keep responding, plus update once we've talked with the manufacturer.

[MarkL]

I spoke with Cooper Eaton and have raised a ticket with them. 

Btw, I wrote:
>  the + and - conductors are not continuous on the loop - each device deliberately intercepts them
This is only partially correct. Apparently, the Loop + should measure as continuous around the loop (very low resistance), while the Loop - should measure as 2.5 kOhms (series) per device on the loop. We'd not tried measuring the loop + and -, but will do as part of the continuing investigation.