PLC output for 120v horn

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travmad804

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I am a newly licensed electrician but still green to a lot of things, espescially when it comes to PLCs. I was given the task of wiring an audible alarm from a 120V output card on the PLC. I know what slot and what output terminal it should land on. Im assuming the PLC will give a signal when the alarm fault shows up for the horn to activate until the alarm is silenced at the HMI manually. My question is do I need a relay or anything in line with the horn? Or is it that when the fault arrives it puts 120V on that terminal directly as an output on the PLC card itself and that is enough to sound the horn alone?
 

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Depending on the type of horn you may want to use an interposing relay. A 120VAC claxon type of horn can be electrically noisy.
 
If I'm the electrician and have that drawing, I'm expected to wire it per the drawing. If I'm smart and see that it draws more current than the module is rated to supply, I should request an updated design/print. Trying to troubleshoot undocumented field modifications is difficult.
 
Adding a relay may lesson your liability if something goes haywire in that panel, relays draw next to nothing, but isolates the horns load to that particular really, JMO
 
someone should verify that the contacts are rated for the specified horn. ask engineering or whoever gave you that drawing to verify. best practice is an interposing relay.
 
drktmplr12 said:
someone should verify that the contacts are rated for the specified horn. ask engineering or whoever gave you that drawing to verify. best practice is an interposing relay.
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Yes, if something gets ugly on the horn side of the circuit, you're not replacing the PLC board.
 
petersonra said:
who determined that was "best practice"? There is nothing wrong with hooking up an ac horn to a triac output.
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Anyone who's had to replace a fire alarm motherboard because they decided to use the on-board relays and someone made a downstream connection that they shouldn't have, that's who. Just sayin'.
 
petersonra said:
This is not a junk fire alarm.
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So, that's not exactly a snappy come back. Panels built to UL864 are rarely "junk", and no fire alarm designer or installer will take the risk of using the on-board relays, unless they are used as pilot relays for beefier outboard components.
 
I would clarify that they want the horn tied directly to the output before I wired it direct to the output as the drawing is showing.

I understand the advantages of an interposing relay, but I would NOT install an interposing relay without bringing it up to those that gave me the drawing first.
 
Cow said:
I would clarify that they want the horn tied directly to the output before I wired it direct to the output as the drawing is showing.

I understand the advantages of an interposing relay, but I would NOT install an interposing relay without bringing it up to those that gave me the drawing first.
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A good point, and I would definitely want to run that by the designer. So, send an RFI or install as shown.
 
gadfly56 said:
So, that's not exactly a snappy come back. Panels built to UL864 are rarely "junk", and no fire alarm designer or installer will take the risk of using the on-board relays, unless they are used as pilot relays for beefier outboard components.
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Fire alarm panels are built and tested to be very reliable when installed and used exactly as intended. Their use is restricted to a very narrow set of components and wiring methods, so the panels don't need much protection for the I/O points. PLCs are built to be used in a wide variety of industries and applications, so their I/O tends to be more robust, as it has to withstand noise, electrical faults, wiring and troubleshooting errors, etc. They also cost more than a fire alarm panel typically.
 
jusme123 said:
Adding a relay may lesson your liability if something goes haywire in that panel, relays draw next to nothing, but isolates the horns load to that particular really, JMO
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I would argue that not wiring per the prints will increase your liability if something goes wrong. Yes, it is prudent to question things that don't look right, but to install things because that's how you think it should be defeats the purpose of paying someone to design the system and draw up prints.
 
cpickett said:
I would argue that not wiring per the prints will increase your liability if something goes wrong. Yes, it is prudent to question things that don't look right, but to install things because that's how you think it should be defeats the purpose of paying someone to design the system and draw up prints.
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Most certainly true. If the OP thinks an interposing relay should be installed, he should definitely send an RFI to the engineer. In my experience, the engineer, if the change is approved, will add something like "additional components to be installed at no additional cost to the owner" to the reply. So, if approved, it's likely to be on the OP's nickel.
 
cpickett said:
I would argue that not wiring per the prints will increase your liability if something goes wrong. Yes, it is prudent to question things that don't look right, but to install things because that's how you think it should be defeats the purpose of paying someone to design the system and draw up prints.
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I would agree it would increase your liability.
Most engineers I've dealt with are open to questions and suggestions particularly from site installer because there are times, whether its other trades or structure issues arise, where "as drawn" either don't work or are in conflict. They will as gadfly56 indicated will amend drawing, if deemed necessary. Large projects will have many engineers doing drawings on different aspects not just one and sometimes I've seen them not in sinc ie hvac/electrical, plumbing/electrical, structural/electrical. (Had one where structual had a door in a space electrical showed a switch, project had hundreds of pages of prints, even though we had overlays for comparison, things got missed.)
Important particularly on large projects where many levels of inspections will take place, all referring to the prints and engineering specs. The CO for building could be held up in event of nonconforming installations, not to mention non issuance of an insurance policy. I've been involved with a larger project where an insurance inspector also showed on site occasionally, (in addition to the AHJ and Building inspector), seeming to check on compliance to engineering.
 
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