Wiring Auxiliary contacts & Shunt trip to Digital Input/output terminals

Hello guys,

I will try to summarize my problem as follows

Problem:
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I want to monitor the status (& also remote control) of a UL489 miniature circuit breaker (MCB) in some control panel. The MCB brand is Weidmuller & will be equipped with auxiliary contact & Shunt trip. The control unit is Beckhoff IPC Cx.

The shunt trip data sheet mentioned that " rated current at 24 vdc is 2A" , "minimum response power 30 VA"
The auxiliary contact data sheet mentioned that " rated current at 24 vdc is 6A"
Beckhoff's Digital input terminals (ELxxxx) mentioned that " rated input current for each channel is 3 mA"
Beckhoff's Digital output terminals (ELxxxx) mentioned that " max output current for each channel is 500 mA"

Question:
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1) Monitoring : if i wired the NO auxiliary contact to the terminal's channel the drawn current will NEVER be larger than 3 mA , now if the available power supply is 1A 24 vdc ( the power supply will serve other small calculated loads)...It will be enough , correct ? i.e " rated current 6A at 24 Vdc" means maximum "possible" current but won't be necessarily be drawn , if so , why would it need to be that large current it is designed for the purpose of monitoring ?!

2) control : i think i can't wire the shunt trip directly to the DO terminal channel , the min. response power is 30 VA , so Imin = 30 / 24 = 1.25 A hence the terminal can't supply that current & the device won't trip remotely , so i have to use a relay (mechanical or solid state) with different power supply to the relay with ampere higher than 1.25A , correct ?

3) protection : is it a good practice to use some sort of fusing (like 0.5A fuse) for each channel in the Digital input & also digital output terminals? and is i am going to use relays for the Digital output terminals , would i still need fuses or the relay would isolate both parts (input to relay & output of relay ) or should i fuse after the relay output to protect the relay ?

Thank you all in advance & please excuse me for being the first time wiring shut trips & monitoring circuit breakers inside panels.

Hi mhismail,

1) Monitoring : if i wired the NO auxiliary contact to the terminal's channel the drawn current will NEVER be larger than 3 mA , now if the available power supply is 1A 24 vdc ( the power supply will serve other small calculated loads)...It will be enough , correct ? i.e " rated current 6A at 24 Vdc" means maximum "possible" current but won't be necessarily be drawn , if so , why would it need to be that large current it is designed for the purpose of monitoring ?!

Click to expand...

Yes you are correct that each input channel will use about 3mA of current. Weather or not a 1A supply is enough is going to be a function of all your loads added together. For longevity and future expansion I wouldn't use a power supply that is smaller than 120 - 150% of your current load.

2) control : i think i can't wire the shunt trip directly to the DO terminal channel , the min. response power is 30 VA , so Imin = 30 / 24 = 1.25 A hence the terminal can't supply that current & the device won't trip remotely , so i have to use a relay (mechanical or solid state) with different power supply to the relay with ampere higher than 1.25A , correct ?

Click to expand...

You are correct in your thinking. Normally I always have my PLC outputs connected to a relay. It is much cheaper to replace a relay that has gone bad than a PLC Output card. Terminal Block Relays from AB or Phoenix make it really easy to use relays without taking up much room in the panel.

3) protection : is it a good practice to use some sort of fusing (like 0.5A fuse) for each channel in the Digital input & also digital output terminals? and is i am going to use relays for the Digital output terminals , would i still need fuses or the relay would isolate both parts (input to relay & output of relay ) or should i fuse after the relay output to protect the relay ?

Click to expand...

Normally I will provide the power for inputs via fuse blocks. That way if you have a bad wire only one input will loose power and not the whole card of inputs. You do have to be careful that you wire and program your input devices, especially items that can affect safety in a "fail safe" condition. In other words power on or 1 is a good condition and power off or a 0 is a fault condition. That way if you loose a fuse your program will notice it and stop any operations that can be unsafe.
As for outputs I normally just put one fuse to provide power to the output module that is sent to the relay coils. The chances of any one of the relays shorting out is small. On the power for the contacts on the relay out to the field it will really depend on what I am working with. Many times I have a mix of AC and DC on the relays so the power will come from many different sources. If you have the room in the panel and know what you are going to be powering you could put a fuse in for each relay output. If not I would put one fuse in that will cover the max current for the relays you are using.

Hope this helps.

Bullzi has some great points which I would like to add on. Depending on your application size, I would recommend using circuit breakers over fuses, that way if it trips, you just push the breaker back in, instead of replacing fuse.

My idea is use circuit breakers for smaller amperage items and fuses for bigger amperage items, like 10A or more.

And yes, always place a fuse/circuit breaker in between your PLC and output device. Some PLCs have internal fuses in them already, but I still add my own into a panel also.

Bullzi said:

Hi mhismail,


Yes you are correct that each input channel will use about 3mA of current. Weather or not a 1A supply is enough is going to be a function of all your loads added together. For longevity and future expansion I wouldn't use a power supply that is smaller than 120 - 150% of your current load.



You are correct in your thinking. Normally I always have my PLC outputs connected to a relay. It is much cheaper to replace a relay that has gone bad than a PLC Output card. Terminal Block Relays from AB or Phoenix make it really easy to use relays without taking up much room in the panel.



Normally I will provide the power for inputs via fuse blocks. That way if you have a bad wire only one input will loose power and not the whole card of inputs. You do have to be careful that you wire and program your input devices, especially items that can affect safety in a "fail safe" condition. In other words power on or 1 is a good condition and power off or a 0 is a fault condition. That way if you loose a fuse your program will notice it and stop any operations that can be unsafe.
As for outputs I normally just put one fuse to provide power to the output module that is sent to the relay coils. The chances of any one of the relays shorting out is small. On the power for the contacts on the relay out to the field it will really depend on what I am working with. Many times I have a mix of AC and DC on the relays so the power will come from many different sources. If you have the room in the panel and know what you are going to be powering you could put a fuse in for each relay output. If not I would put one fuse in that will cover the max current for the relays you are using.

Hope this helps.

Click to expand...

thank you so much..very descriptive and clear answers. I will re check the power supply calculations to make sure it has enough safety factor.

thank you for your help and fast reply