VRD stands for voltage reduction device. When a VRD is fitted to a welding machine it reduces the maximum unloaded open circuit voltage across the output terminals of the welder to a safe voltage.

When a load of less than 200 ohms resistance is applied to the output of the welding machine the VRD will sense this and turn the welder to full output then welding will commence.

When the resistance across the output rises above 200 ohms or the electrode is removed from the workpiece then the VRD will turn the output of the welder back to a reduced output state.

Why is this necessary?

A lot of new welders and some older units have an open circuit voltage in excess of 105 volts D.C. or 85 volts a.c. This voltage is currently legal and in normal (dry) conditions may be quite safe.

The only trouble is 95% of sites do not have these normal conditions e.g. (damp or wet, caustic, salty, working at height or in a confined space) an electric shock in these conditions can and has proven fatal. (Ventricular fibrillation, falling from heights and inability to escape the power source when in confined spaces) not to mention the serious physical injuries sustained as a secondary result of an electric shock. Electric shock from a welder power source can also cause serious internal injuries to the human body.

Australian Standards AS1674 Categories of Welding Environments

Category A environment

An environment where the risk of an electric shock or electrocution by arc welding is normal work practice is used and it is not possible for a welder or any other worker to be in contact with the workpiece, in the event of being in contact with a live part of the welding circuit.

Maximum Permitted Open Circuit Voltages – d.c. 113 V peak, or a.c. 113 V peak and 80 V rms

Category B environment

An environment where there is a significant risk of the welder contacting the workpiece or other parts of the welding circuit. NOTE: Such an environment may be found where the ambient temperature is less than 32°C and freedom from movement is restricted, so that an operator is forced to perform welding in a cramped position (e.g., kneeling, sitting, lying), with physical contact with conductive parts (e.g., the workpiece); or (b) there is a high risk of accidental or unavoidable contact by the operator with conductive elements, which may or may not be in a confined space as defined in AS/NZS 2865. 1.3.6.3

Maximum Permitted Open Circuit Voltages – d.c. 113 V peak, or a.c. 68 V peak and 48 V rms

Category C environment

An environment where the risk of an electric shock or electrocution by arc welding is greatly increased due to low body impedance of the welder and a significant risk of the welder contacting the workpiece or other parts of the welding circuit. NOTE: Low body impedance is likely in the presence of water, moisture or heat, particularly where the ambient temperature is above 32°C. In wet, moist or hot locations, humidity or perspiration considerably reduces the skin resistance of human bodies and the insulating properties of personal protective equipment accessories and clothing.

Maximum Permitted Open Circuit Voltages – d.c. 35 V peak, or a.c. 35 V peak and 25 V rms