Value and purpose of Class 2 circuit protectors

[Gabe1234s]

I'm a beginner automation designer working on my first independent project and I'm hoping to get some advice on class 2 circuit protectors such as this one:

http://ab.rockwellautomation.com/Ci...ion/1692-Electronic-Circuit-Protection-Module

I've been given conflicting advice to use or not use such a device.
What kind of protection does this device offer and in what situations would it wise to use it?

The project I'm working on uses a Rockwell L16 PLC which needs to handle 13 digital inputs and 17 digital outputs. Inputs include pushbuttons and proximity sensors and outputs include solenoid air valves, 1 pull solenoid, power relays, and a bunch of LED indicator lights.

 

[Bullzi]

Hi and Welcome to the Forum,
A Class 2 circuit will have a limited amount a energy. This limit is designed to protect personnel from electrocution and prevent a fire in the event of a short circuit.

You will normally see that a Class 2 circuit is required on Instrument Power for PLC and HMI's. You dont normally want to power your field devices with a Class 2 Power Supply.

As to use or not use a Class 2 Power Supply as I understand it: If the device says that it is to be powered by a Class 2 Circuit then you need to use a Class 2 Circuit.

Now there are a couple different ways to get a Class 2 Circuit.
1. You can use a Class 2 Power Supply.
2. You can use a DC to DC Converter.

I tend to use the DC to DC converter. Class 2 Power Supplies by design are limited to about 7.5A max. So if you need more power than that you end up putting in 2 power supplies. I usually will put in 1) 20A power supply and then add a DC to DC converter that is Class 2. Just makes it simpler.

In your case the L16 Instrument Power should be Class 2 but the Field Power dosen't have to be Class 2.

Hope this helps.

Edit: Here is a link to a good paper about Class 2 Circuits http://www.staveb.ch/downloads/1692-wp001_-en-p.pdf