Hi all im trying to make a phase failure alarm for a 3 phase refrigeration unit. im struggling with what parts i need and how to get the low voltage output to trigger the siren

 

Use a relay rated for ur operating voltage.

 

Like R!f@@ said, if 240V Id use 3 relays 1 for each phase to neutral so contact closes if any one phase goes down & trigers the siren.

 

Hi all im trying to make a phase failure alarm for a 3 phase refrigeration unit. im struggling with what parts i need and how to get the low voltage output to trigger the siren

Can you say what type of three phase voltage powers the unit is it WYE or is it DELTA?

 

Like R!f@@ said, if 240V Id use 3 relays 1 for each phase to neutral so contact closes if any one phase goes down & trigers the siren.

Phase failure protection is a little trickier than you might think. For example, if a 3 phase motor is running, and one phase goes down (perhaps a fuse blows), a relay connected from line to neutral, even on the dead phase, will remain energized due to a voltage from the other phases via the motor windings.
Though I'm sure many on this forum could tackle this problem, many solid state devices today can provide this protection, along with overload protection if desired.

 

a current transformer around each phase could work. As Hagen mentioned, if your load is a 3ph motor or heater, voltage from the other 2 phases will be present on the 3rd phase, but if there is an open, no current will be flowing & the transformer will detect this. I also think a current transformer would be a better option since it's analog, and you mentioned wanting a "low voltage" indication. If you know how many watts your equipment draws, then you could translate a drop in current to a drop in voltage.

something like this

 

Google is your friend..... http://www.drillspot.com/products/53143/Square_D_8430MPDV29_Three_Phase_Failure_Relay

 

i was planning on linking someting out of the 3 phase isolators there are 2 refridgeration units fed seperatly, they only draw a low current 15a or so. i only really need to know if the power fails to the units rather than an actual phase drop i suppose. im out of my depth a bit with this..

i could do with a simple wiring diagram.??!! this is all a bit technical for a domestic electician

thanks for all your inputs though .. muchly appreciated

 

its delta that powers the refridgerator units

 

Google is your friend..... http://www.drillspot.com/products/53143/Square_D_8430MPDV29_Three_Phase_Failure_Relay

Whats wrong with this?
it works with wye
it trips on undervoltage, overvoltage, and phase loss - pretty much any kind of power failure.
Only thing I see is your amperage requirement. you got your keywords now "3 phase line monitor relay". Shouldn't be too hard to find one that fits your amperage needs.

as far as the wiring diagram, you should have 3 big wires going to the reefer unit (isolate power) cut these, insert relay.