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Health, safety and accident prevention: electrical hazards - power source and installation

 

Guidelines are given on the principal health and safety considerations to ensure safe welding practices and prevent accidents. The hazards associated with the use of electrical equipment are highlighted.

The arc welding circuit

MMA and TIG processes can be either alternating current (AC) or direct current (DC) whilst MIG is usually only operated with DC. As arc processes need a large current (up to 500 A) but at a relatively low arc voltage (10 to 40V),the high voltage mains supply (230 or 400V) must be reduced. In its basic form, the power source for generating AC comprises a transformer to reduce the mains voltage and increase the current for welding. For generating the DC arc, a rectifier is placed on the secondary side of the mains-fed transformer or alternatively, a motor- or engine-driven generator can be used.

Welding installations

jk28a.gif

Typical arc welding installations for both single and multi-welder operations are described in HSE guideline No 118, Electrical Safety in Arc Welding. When the welding circuit is connected, the following guidelines should be adopted:

  • the connection between the power source and the workpiece should be as direct as practicable
  • use insulated cables and connection devices of adequate current-carrying capacity
  • extraneous conductive parts should not be used as part of the welding return circuit unless part of the workpiece itself
  • the current return clamp should be as near to the welding arc as possible

When attaching the welding current and current return cables, it is essential that an efficient contact is achieved between the connection device and the workpiece to prevent overheating and arcing. For example, current and return clamps must be securely attached to 'bright' metal i.e. any rust or primer coatings should be locally removed.

Power source and earthing

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The normal practice in the UK has been to provide a separate earth connection to the workpiece, ( Circuit a) so that, in the unlikely event of an insulation breakdown between primary and secondary circuits, the fuses will blow. However, the separate earth connection increases the risk of stray currents which may damage other equipment and conductors.

As modern power sources have been designed to have a much higher level of insulation ( double or reinforced insulation), a separate earth connection is not recommended ( Circuit b).

There is a potential problem in that both designs can often be found in the same welding shop. The newer power source can be identified by the power source's Rating Plate, which indicates it has been manufactured to recent standards e.g. EN 60 974-1 or IEC 974-1.

In very old designs, the welding circuit was sometimes connected internally to the power source enclosure ( Circuit c). However, the danger is that even with the welding return lead disconnected, and a separate earth connection, welding is possible with current flowing through the earth. Because of the risk of damaging protective earth and other connectors, this type of power source is considered to be obsolete and should not be used.

Manufacturer's rating plate

The following symbols are used on the manufacturer's Rating Plate to indicate the type of protection:

jk28c.gif

Electrical hazards

Electric shock

In all manual arc welding operations, the principal risk is from electric shock, most likely from contact with bare live parts of the welding circuit. The arc voltage is within the range 10 to 40V, but, as the voltage required to strike the arc may be substantially higher, power sources have an open circuit voltage (OCV), typically up to 80V. Although these voltages appear low compared with the 230V domestic mains supply, work by the International Electrotechnical Commission shows that only voltages below 50V AC or 120V DC are unlikely to be dangerous to healthy people in a dry working environment. In other conditions such as restricted conducting locations or wet surroundings, potentials of 80V AC have been the cause of fatal electric shocks. Devices are available which reduce the open circuit voltage when welding is not being carried out. These voltage reduction devices (VRD) should meet the requirement of BSEN 60974-1.

Appropriate protective clothing such as gloves, boots and overalls will protect the welder from electric shock.

Stray welding currents

A different kind of electrical hazard can arise from stray welding currents which return to the welding transformer by paths other than the welding return lead. For example, although the return is disconnected, welding is possible when the return current flows through:

  • protective earth (PE) conductors of other electrical equipment, or the power source itself
  • wire ropes, slings and chains
  • metal fittings and pipework
  • bearings in motors

Damage to the PE conductor in particular could mean the equipment is no longer being earthed.

Stray currents may be substantial and comparable with the welding current level where there is poor or faulty insulation of the return lead which may be short- circuited by other conductors. When welding on building structures and pipework installations, the welding return lead should be placed as close as possible to the point of welding. The exception is where the metallic grid, support structure or metallic ship hull is used as part of the welding return circuit.

There is an increased risk of stray currents when welding on structures which have an inherent connection to earth such as ships or pipelines. There could be an unacceptable indirect current return path with damage to conductors if, for example, the current return lead is detached and the circuit becomes open. The recommendations for the electrical distribution systems and earth connections in various situations are described in HS(G) 118.

Safe practice and accident avoidance

Welding equipment should conform to the appropriate standards (as listed at the end of the article); electrode holders that are insulated overall are recommended so no bare metal can be inadvertently touched.

Welding leads and return leads should be insulated and thick enough to carry the current safely; connectors should also be insulated to avoid inadvertent access to live conductors and adequate for the current being carried.

The welding return lead should be connected as near as practicable to the welding arc; metal rails, pipes and frames should not be used as part of the welding circuit unless they are a part of the workpiece itself.

Check the workpiece earthing requirements. When using a double or reinforced insulation power source, stray currents can be avoided by not earthing the workpiece or the welding output circuit.

The welding leads, connection devices and electrode holder or torch should be checked at regular intervals for 'fitness for use'; repair or replace damaged or worn components.

Publications and relevant standards

  1. HS(G) 118 'Electrical Safety in Arc Welding', HSE Books, 1994
  2. 'The Arc Welder at Work', Welding Manufacturer's Association
  3. BS EN 60974-1: 2005, Arc Welding Equipment. Power Sources.
  4. BS EN 470-1: 1995, 'Protective Clothing for Use in Welding and Allied Processes'
  5. BS EN 60974-7: 2005, Arc Welding Equipment. Torches.
  6. CLC/TS 62081:2000 'Arc Welding Equipment - Installation and Use' (IEC62081:1999)
  7. BSEN 60974-6:2003 'Arc Welding Equipment - Limited duty manual metal arc power sources'.
  8. BS 638-4:1996 'Arc welding power sources, equipment and accessories. Specification for welding cables.'
  9. BS 638-5:1988 'Arc welding power sources, equipment and accessories. Specification for accessories.'
  10. BS EN 60974-11:2004 'Arc welding equipment. Electrode holders.'
  11. BS EN 60974-12:2005 'Arc welding equipment. Coupling devices for welding cables.'
  12. BS EN 60974-8:2004 'Arc welding equipment. Gas consoles for welding and plasma cutting systems.'

The information was prepared in collaboration with Roger Sykes, Health & Safety Executive and Geoff Melton, Chairman, BSI WEE-6 Committee.

This Job Knowledge article was originally published in Connect, June 1997. It has been updated so the web page no longer reflects exactly the printed version.

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