How to Determine Wiring Configuration

2 Wire Devices

When wiring a Digital Input we are only interested in 2 wires as it is purely a switch contact, but if your cable has 3 wires which 2 of the 3 wires do we need ? . . .

This largely depends on what it is that you would like to monitor, for example . . . you may have a pressure switch gauge, the switch gauge may have 2 or 3 wires, if it has 2 wires then you will need to make sure that the switch gauge is of the correct switching type, a switch gauge may change state when it falls below a mechanical setpoint (descending) for Fail Safe operation the switch gauge would need to go Open Circuit as it falls past the setpoint on Descending, You would use this type of gauge to monitor Low Cylinder Pressure for example, But what if your gauge is normally Open Circuit and goes Closed Circuit when it falls below the setpoint can you still use it ? With the ALU yes you can and all you would need to do is set that input channel to be N/O (Normally Open) instead of the default N/C (Normally Closed) but there is a downside to setting any input up as Normally Open which is if someone was to disconnect the Input or cut through the wire you would have no indication of this so for this reason we would never recommend that you set any input up as Normally Open if it is a safety device, When Inputs are set up as a Normally Closed input then if there is any disconnection / fault of the device or the wire is cut then the monitor will Alarm that the input has a fault and also the Output Relay will change state.

NEVER SET A SHUTDOWN (High Level) ALARM AS N/O (Normally Open) ALL SHUTDOWN INPUTS AND SAFETY DEVICES MUST BE CONFIGURED AS NORMALLY CLOSED (N/C) (Fail Safe)

To summarize - A 2 wire pressure switch gauge must be of the correct type (Normally Closed which Opens below the setpoint (Descending) to monitor Low Pressure, OR Normally Closed which opens above the setpoint (Ascending) to monitor High Pressure or Over Pressure

3 Wire Non Powered Devices

These are devices that do not need to be powered - for example a mechanical switch, a reed switch, reed switch pressure gauge, door switch, you will need to consult the documentation of the device.

If for example you have a non powered pressure switch gauge but it has 3 wires which 2 wires do you need ? This depends on the switch gauge as some have the option of monitoring above OR below a setpoint (you have the choice)

If this is the case and you do not have the documentation you will need to establish which 2 wires you are interested in, You know that it is preferable to have the device working Fail Safe (Normally Closed) with a 3 wired non powered switch gauge you will have one wire that is the common (this is one of the wires you need) and then you only have one other wire to determine, bear in mind that you will need your device to go above and below your mechanical setpoint (if it is a gauge you may be able to unscrew the front to carefully move the needle above and below the setpoint, otherwise you may need to pressurize the device to achieve the same action) We will use a Wika contact switch gauge for our example to demonstrate our tests . . .

The switch gauge should have documentation regarding wiring colours or a label on the gauge itself

For Wika contact switch gauges . . .

if you want an alarm to activate when the needle rises ABOVE the mechanical setpoint you will need to connect the Brown (Common) and the Black (signal) wires to the required Input (0v & I/P)

if you want an alarm to activate when the needle falls BELOW the mechanical setpoint you need to connect the Brown (Common) and the Blue (signal) wires to the required Input (0v & I/P)

As previously described you are looking for the correct two wires for your required operation.

If you want an alarm below the setpoint you need the two wires that go open circuit as it the needle falls below the setpoint, if you are looking for an alarm above the setpoint you need the two wires that go open circuit as the needle rises above the setpoint, when checking the different combinations of wires you will find that one of the wires is required in both cases - this is the Common wire (in this case Brown)

With a multimeter set for Resistance/Continuity/Tone with your established Common wire attached to one multimeter probe find the wire that gives you a continuous signal with the sensor in a good healthy condition (in our example 'above' the setpoint but it is the same exercise if you want an alarm 'below' the setpoint)

With the same two wires still connected to your multimeter probe put your sensor into Alarm fault condition (in our example 'below' the setpoint but it is the same exercise if you want an alarm 'above' the setpoint) you should notice that you lose the continuous signal, this means it is Open Circuit - You have found the two wires that you need to connect to your monitor !

Basically you are looking for Closed Circuit when the sensor is in good healthy condition and Open Circuit when the sensor is in fault alarm condition

3 Wire Powered Devices

These work in a very similar way but your sensor may have an indication LED and may work on a logic basis, this is the reason that it requires power, the monitor provides you with limited power for this purpose, you will need to consult your sensors documentation. Our test have been done with Span/Millipore/Brookes IPT122 pressure switch gauge with integral indicator LED and works perfectly

The power limitations for powered sensors . . .

A maximum total current of 150mA is provided by the controller for customer devices.

For Span Millipore / Brookes gauges (with led) . . .

Connect the Green wire (signal) to the I/P of the required Input

Connect the Black wire (Ground) to the 0V of the required Input

Connect the Red wire (Power) to the +24v of the required Input

To change whether the alarm triggers when it goes above or below the setpoint you will need to remove the back of the switch gauge and change the position of the internal jumpers (one is for the led and one is for the switch, you will need to change both jumpers for correct operation.

Compatibility

Our monitors are compatible with normal Dry Contact switches, PNP/NPN Logic and NAMUR specification inductive proximity switches.