Wiring A 3 Phase Fail Relay To An Ab Input Card

TURG

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Join Date
Jul 2008
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TEESSIDE
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Hi
I am needing some advice on how to wire a 3 phase fail relay into an Allen bradley input card. And also what line of logic should I use to bring in an alrm when the relay switches.
The situation is: I have a mains power supply that is linked to a back up UPS, if mains power is lost. The relay has been wired up so that if the mains power is lost and we go over to the UPS, the relay switches over from being NC to NO, this triggers an alram on a SCADA screen notifying an operator. How do you wire the 3 wires to the AB Input card? The 3 wires are: A Normally open, a normally closed and a common.Also how would you write the line of logic to make sure when the mains power is on and working the relay is ok (NC), but when mains power is lost, the relay switches to being NO and an alarm is activated. I can set up the alarm ok. Its just knowing what initial line of ladder i need to write.

Many Thanks
 
What three wires?
You have a contact, it is either on or off.
Wire (the proper) source voltage to one side (say, Common), and the other side gets wired to a single (voltage compatible) input. I don't know what PLC you are using, but your logic might start like:

XIC I:3/2 OTE B3:15/0
 
Turg:

Connect your relay coil to you AC suply ( 230, 380, 120, 460 etc), use a N.O. contact of your relay ( 2 wires) and connect it to one your inputs of your plc (either Ac or DC) so that when AC is present your N.O. contact is close, when there is a power outage your relay will be deenergized and you will notice because now the contact is open, use this input and put it on your scada so you know when your plc is receiving energy from ac suply or from your ups. In conclusion you need two wires not three.
 
No offense intended, but NC contacts do not switch over to NO (or vice-versa). The relay simply opens or closes and the contacts change state.

So the real question is when all 3-phases of power are present then is the relay energized or not? It probably is energized, so the NO contact would turn ON a PLC input when everything is OK.

A NC contact would turn ON a PLC input when a phase is lost.

I'd use only the relay's NO contact and my PLC descriptorn would be "3-Phase power is OK". That should make it fail-safe where even if a control wire breaks you will lose the input and get an alarm.

Pick up a book on how to actually wire PLC inputs and it will help out your programming skills.
 
3 phase

wiring a simple relay across 2 of the 3 phases is a poor optiton. you need a " phase monitor " this device looks at all 3 phases for proper rotation low or high voltage, or total loss of power. most have a potential free form C relay output that you will will wire to the input of the PLC
 
John Hawkins said:
wiring a simple relay across 2 of the 3 phases is a poor optiton. you need a " phase monitor " this device looks at all 3 phases for proper rotation low or high voltage, or total loss of power. most have a potential free form C relay output that you will will wire to the input of the PLC

yes, you are right, but I was trying to answer turg´s question.
 
HI thanks for the replys. the input module is an AB 1794-ib16 input sink card. So you suggest using the NO (XIC) wire of the relay for the input of the card. So when the phases are ok this contact will run true, but when a phase is lost the contact will run false. What do you then do with the NC wire and common. I tried looking in my uni work and on goole etc, but no real help on hard wiring PLC I/O cards.
Hey I go to uni and when you get out into the big world a work its alot different.
Thanks for the tips.
Mark
 
You don't do nothing with the NC contact and the common goes to the COM of the input card.
 

(NC)o------X Not used
/
(COM) -----o
Connect
To Hot (NO)o------> To PLC Input
Source (Sinking)


While 3-phase is OK the relay will close the NO contact.
While PLC Input is ON, the 3-Phase is OK.

If PLC Input is NOT ON then 3-phase has failed.

3-Phase 3-Phase
Input is OK
---| |--------( )


-OR-

3-Phase 3-Phase
Input Failure
---|/|--------( )


 
TURG said:
HI ,

Thanks for the reply and help.
What do you mean by the phrase "Connect to HOT SOURCE"?
Thanks again.

Sir,
With all due respect, you need to get someone involved to who actually understands electrical work.

If you are asking this question then you are not qualified to do the work you are attempting to do.
 
have to agree with dogleg43 on this...if the terminology has you confused...

on a side note, is the scada linked to the plc? are they both on the same network? just some starting points.
 
Hi
I belive we have been lost in translation.
In England we refer to "hot contact" as "live", which I understand.
Crazy how same things are named differently depending on where you are from.
The SCADA and PLC are linked to the same network.
Thanks
 
TURG said:
Hi
I belive we have been lost in translation.
In England we refer to "hot contact" as "live", which I understand.
Crazy how same things are named differently depending on where you are from.
The SCADA and PLC are linked to the same network.
Thanks

Understood.
Here's a neat website with all kinds of American slang terms for the electrical field http://www.tradeslang.com/
 

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