Automatic Detector Removal

[Wiz]

Oh various sages of this forum please answer this anomaly that keeps me awake most nights at my current abode of the local Home For The Bewildered;!

BS5839 Part 1 2002 clearly states that where detectors are designed to be removed from their bases....removal of any detector(s) from the circuit should not affect the operation of any MCP......

I am probably wrong, but I seem to recall that sometime after that particular requirement was first added to a previous version of the BS, an amendment was subsequnently issued that required that such action should also not affect other automatic fire detectors as well as MCPs.

If my memory is correct, why has it changed back again?

And, whether my memory is good or bad, why are the continued operation of detectors in these circumstances not deemed to be as important as MCP's in the 2002 version?

(I appreciate that the shottky diode method will keep the integrity of the circuit complete anyway, but there could be other methods devised that met the recommendation, but disabled other detectors)

Thanks in advance for any input you can provide. Once solved, I have numerous other 'things that make my brain hurt' to ask you!

[Graeme]

New one on me.

Always been MCP.

You may be getting mixed up with Clause 12.2.2 h-removal of any mcp or detector should not affect the operation of any fire alarm device.

[Wiz]

New one on me.

Always been MCP.

You may be getting mixed up with Clause 12.2.2 h-removal of any mcp or detector should not affect the operation of any fire alarm device.

Well, as I said, my memory could be flawed.

Any thoughts why they don't think it is as important to keep detectors working?

[Graeme]

one reason maybe is humans are more effective than afd but need a ways of raising the alarm

[kurnal]

Oh wiz
What you gone and done that for? Now I will be kept awake wondering what a shottky diode is. And just as I have finally  mastered the spelling of carbon tetrachloride in my fire safety recommendations.

[Wiz]

Oh wiz
What you gone and done that for? Now I will be kept awake wondering what a shottky diode is. And just as I have finally  mastered the spelling of carbon tetrachloride in my fire safety recommendations.

Oh Kurnal,
Mr Shottky must know how to spell his own name and that's how he spelt it when he wrote to me recently seeking my thoughts on forward voltage drop. Or is this another trick of my memory and imagination? Kurnal, come join me in this Home For The Bewildered. The nice ladies looking after me might let you have some of my medicine to help you sleep, and all washed down with lashings of dihydrogen oxide!

But do you have any thoughts about the original question!

[Wiz]

one reason maybe is humans are more effective than afd but need a ways of raising the alarm

Graeme thanks for your input. I know what you mean and can see the absolute importance of keeping mcps functioning whilst that most effective automatic detection device, the human being, is around to detect a fire. But I'm still wondering why the cop didn't include afd in the recommendation (especially as they will continue functioning in the most common method of detector removal monitoring) ?

[Graeme]

mcp's are more likely to operate on less voltage from dropping when a detector is removed

[Wiz]

mcp's are more likely to operate on less voltage from dropping when a detector is removed

Yes Graeme, I think you are probably right with this answer. I had previously considerd the combined effect of the forward voltage drop of the monitoring diodes when removing detectors, during my nocturnal musings on the subject. But I discounted it because of the low FVD of Shottky diodes, and the wide operating voltage of most modern detectors.

However, I suppose the committee looked at it, and agreed it was more important that mcps continued working rather than afd, if also insisting on the latter, precluded the use of possible different removal monitoring circuits and/or detectors with a smaller voltage operating range.

I personally don't agree with the limited scope of the recommendation, because it seems that keeping all afds working on the removal of any other dectector(s) is easily possible these days and, surely, equally important as the mcps.

Thanks again for your well thought-out inputs. I will now sleep a bit easier, but I have other insomnia-inducing queries that I will put forward for your thoughts when I have recovered sufficiently from this one!

[lucky]

I think you will find the shottkky diode contains coper which will still contract at a lower voltage,when the current is de creased the mcp will operate in some circumstances resulting in another false alarm.

[lucky]

another little one,smoke detectors often go off when they detect steam.

[lucky]

If you throw powder eg talc at a smoke alarm it will go off,a prank often done in care homes or Prisons....

[Wiz]

Oh wiz
What you gone and done that for? Now I will be kept awake wondering what a shottky diode is. And just as I have finally  mastered the spelling of carbon tetrachloride in my fire safety recommendations.

Oh wise and helpful Kurnal, I know I previously gave a flippant reply to your posting on the basis that I found it, as I often do with the jokey parts of your replies, highly amusing. however it occurs to me that the reference to Shottky diodes may indeed have been a serious request for explanation.

If so, and I apologise if I am telling you something you do understand, I will try to give an explanation of the main role in using a Shottky diode in automatic detector removal monitoring because it may help others in the forum and also explain the previous posts of Graeme Millar and myself.

A system of monitoring the removal of detectors from their bases often uses the method of inserting a semiconductor known as a diode in the base of the detector. This diode only has effect when the detector is removed, because whilst the detector is still inserted in the base, it's own internal circuitry short-circuits the diode and keeps the zone wiring circuit connected to devices further along the circuit. When removing the detector from it's base, the diode is no longer short-circuited but it now, itself, provides the continuation of the circuit for the electrical current powering the system to reach devices further along the line.

However many diode types, whilst allowing current to flow through, also cause a drop in voltage. This is known as Forward Voltage Drop and can be as much as around 2V with some diode types. So, as you can imagine, the cumulative effect of unplugging a number of detectors could easily mean that the nominal operating voltage of, say, 24V dc could be far far less by the time it reaches equipment further down the line. It could eventually become so low that it is insufficient to allow that equipment to operate.

The Shottky (sometimes spelt Schottky) type of diode has a low Forward Voltage Drop, typically around only 0.3V, and therefore using this type of diode in the detector bases allows far far more of them to be in circuit, as detectors are removed from bases, before the overall circuit voltage reduces to an unusable level.

I apologise if the above is only 'teaching people to suck eggs'. I'm only trying to help!

[Graeme]

another little one,smoke detectors often go off when they detect steam.

never!!!!

[Wiz]

I think you will find the shottkky diode contains coper which will still contract at a lower voltage,when the current is de creased the mcp will operate in some circumstances resulting in another false alarm.

Lucky, I've never heard anything like this before and I'd love to hear a more precise explanation of the circumstances. Trying to think the scenario through of a typical fire alarm zone circuit, I can only imagine an increase in current from the 'normal' operating situation causing a fire condition. I can't think of any situation where a Shottky diode wired in series with the zone circuit would cause this.