School bell problem.

[ste mac]

I have been asked to look at a school bell system that every now and again the bell will ring & continue to ring until one of the manual bell ring switches is pressed and this then stops the continual ringing. I was shown a timer (Danfoss 841) and there are three override switches dotted about the school (heads office etc), not sure how many bells, contactors, wiring boxes etc.

Having no experience with a school bell system can anyone tell me if there is a standard circuit setup for school bells or are there any circuit diagrams about that may help me with fault finding or are they just as simple as:- timer > contactor + override switches > x amount of bells?

 

[telectrix]

think you're on your own there, mate. just a case of investigation till you find the problem.

 

[lister101]

I think the best way to approach this problem is to draw out a circuit of all the accessible parts of the system & then understand how the manual override interacts with the normal operation of the timed bell. This should point you in the right direction of what is going wrong.

.....Andy;~)

 

[Darkwood]

Bell systems are designed as requested or as per installers unique set-up ... the end results are usually the same but many ways of arriving there from software computer managed, plc, relay, timer, manual switching or a combination of any method ... just get as much info as you can about how its controlled, timed, over-ridden and work from there. Their may be a sticking contactor or relay that sometimes only drops out when current flow is lost this could be when when over-ride is pressed but just a random shot in the dark.

 

[londonlec]

A school I sometimes work in have had a wireless bell system installed, with sounders everywhere that plug into 13A sockets.

Maybe an option if you get stuck.

 

[gnuuser]

Bell systems are designed as requested or as per installers unique set-up ... the end results are usually the same but many ways of arriving there from software computer managed, plc, relay, timer, manual switching or a combination of any method ... just get as much info as you can about how its controlled, timed, over-ridden and work from there. Their may be a sticking contactor or relay that sometimes only drops out when current flow is lost this could be when when over-ride is pressed but just a random shot in the dark.

i agree this sounds like a stuck contactor or relay. (your override would be like our e-stop switches which interrupt current flow to the relays or contactors)

 

[tazz]

Ok danfross is an old unit but still used in most schools. The idea here is the danfross unlike digital timers can switch on for only a few seconds, where timers can only switch at 1 minute intervals. Most Danfross controllers are very expensive for what the are, so we have replaced with din mounted pulse timers within the 24v psu. We can then set for 5 secs of bell duration over 7 times per day and even off during weekends...good bonus is they have auto summer /winter changeover.

 

[grantr37]

here's a user manual I found for the 841


http://www.farnell.com/datasheets/306262.pdf


I think it is worth looking at the relay that switches all the Bells, can you get access on a Saturday for a short while? you could look at the set up in terms of relays and see if you can get one to swap out the old one, it may be that it is starting to stick because of deposits/oxide on the contact points.....if you are still waiting on one arriving, you can pop the cover off and clean it with contact cleaning strips or very very fine sandpaper+ a lens cleaner for Glasses, this will keep it going for a little while longer until a spare arrives...just make sure to disconnect/remove it first

 

[ElleMeFeyo]

Dont have much experience on the matter but my best bet is stuck contactor, sounds like it could be possible.

 

[tazz]

The danfross controls the bells itself with onboard 5amp relay, usually linked to fire alarm panel class change, or seperatly via power supply and its own bell circuit. Its just a bell circuit activated by the control relay, and bells pulling 3ma per bell.
Would suggest removing the Danfross for a few days, and fit a push button and request the manualy used the system and see if the randon bells stops.
80% sure youll find the control unit faulty or incorrectly programmed.