Manufacturers have for years been recommending the use of separate power supplies in safety systems. Part of this was to separate the Safety System from the Controls side of the machine, and this makes sense. If the machine controls go down, you may still want the safety system to be active in order to not expose someone to hazards that may still be present. However, this was not the only reason…

Separation of the Controls and Safety Systems, from a Power Supply point of view also ensures Transient Voltages/Currents do not interfere with safety devices. Electro Motive Force (EMF) has taken down many a safety system by casing all manner of errors/faults to be seen. Complex electronic devices like Light Curtains, Area Scanner, Coded Magnets, Safety Relays and Safety Controllers (Safety PLC’s) not only monitor the 24 Vdc signal for noise (EMF) they also can monitor grounding. Any noise, beyond the design parameters of the device, will cause these devices to fault and shut down (It is a safety system after all...)

So, where does this EMF come from?

Any device can create noise, but VFD’s, Contactors and Welders have been the worst culprits, in my opinion. Drives can generate a lot if inrush when they are engaged. Contactors also create inrush problems, but welders can even interfere with communications (Phone, WFI or Radio) in plants due to the noise they can generate when they are active. Safety products like their power to be clean…

So, what does a Class 2 or a SELV Power Supply do, that others may not?

They not only provide the separation of systems that is part of good safety design, but also provide clean and fast start up power, to safety products. Class 2 Power Supplies limit the amount of current and voltage that can be used in safety circuits. For Class 2 Power Supplies the limit is 100VA. This ensures the power supply also protects against Electrical shock and Fire Hazards, as per NFPA 79. Not all power supplies that are low powered are Class 2, UL has specific requirements in order to attain this rating.

SELV (Separate Extra Low Voltage) Power supplies generally conform to the same performance as the Class 2, but are tested and certified by IEC 60950-1. They are limited to 60Vdc and are designed for Electrical shock protection.

So, how do I know what devices require a Class 2 or SELV Power Supply?

The simple answer is, check the specs or look at the label(see below example)