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Vital Last Resort

Published: 18th Oct 2012

Select the Correct Protective Workwear for Electrical Workers

The steps necessary to correctly specify protective workwear for electrical workers are described by John Maplesden, who discusses the application of the hierarchy of controls, the implementation of mitigation measures and the specification of appropriate workwear for the residual risk associated with the shock and arcing hazards.

Introduction

Alongside any other hazards that the workplace may present, two hazards must be considered in the case of electrical workers. These hazards are electric shock and arcing.

Having identified the hazards of electric shock and arcing, the hierarchy of risk control measures must be applied to reduce the risk presented by the hazards to the lowest reasonably practicable level.

The Hierarchy of Controls is a generally accepted approach to hazard mitigation, with the most effective controls at the top of the pyramid – eliminate the hazard – and the least effective at the base – the use of Personal Protective Equipment (PPE).

Reasonably practicable means balancing the level of the risk against the measures needed to control the risk in terms of money, time or trouble. The decision is weighted in favour of health and safety so that the measures are adopted unless they are grossly disproportionate.

Protective workwear, or PPE, is literally the last line of defence against a hazard and should only be considered after all other risk control measures have been implemented.

The use of PPE is indicated when the residual risk presented by the hazards, after all risk control measures have been implemented, is considered to be unacceptable and therefore warrants its use. In other words, a risk assessment is performed taking into account all of the risks and control measures to determine if PPE is required.

Daniels states that the severity of exposure is used to select the appropriate PPE and that likelihood of exposure is used to determine when it is appropriate to use PPE.

This article discusses how to determine the severity of exposure and the likelihood of exposure to electric shock and arcing hazards in order to determine what is the appropriate PPE, and when it should be worn in any particular set of circumstances.

The shock hazard

This is a dangerous condition associated with the possible release of energy caused by contact or approach to energised electrical conductors or circuit parts.

An electrical worker can receive an electric shock by simultaneously contacting two energised conductors at differing potentials, or by simultaneous contact with an energised conductor and earth (ground).

The effects of electric shock on the body are given in IEC 60479 – ‘Effects of current on human beings and livestock – Part 1: General Aspects’.

A voltage as low as 50 volts applied between two parts of the human body causes a current to flow that can block the electrical signals between the brain and the muscles. This may have a number of effects including:

• Stopping the heart beating properly

• Preventing the person from breathing

• Causing muscle spasms

The exact effect is dependent upon a large number of things including the voltage level, which parts of the body are involved, how damp the person is, and the length of time the current flows.

When an electrical current passes through the human body it heats the tissue along the length of the current flow. This can result in deep burns that often require major surgery and are permanently disabling. Burns are more common with higher voltages but may occur from domestic electricity supplies if the current flows for more than a few fractions of a second.

People who receive an electric shock often get painful muscle spasms that can be strong enough to break bones or dislocate joints. This loss of muscle control often means the person cannot ‘let go’ or escape the electric shock. The person may fall if they are working at height or be thrown into nearby machinery and structures.

The arcing hazard

Where electrical arcing occurs, perhaps as a result of accidental short circuit, the heat generated can be intense and, even if it persists for only a very short time, exposure to an arc can cause deep-seated and slow-healing burns. Engineers and craftsmen often fail to appreciate the very real risk of injury that can arise from arcing. As a result, there are several hundred serious burn accidents each year arising from unsafe working practises. The intense ultraviolet radiation from an electric arc can also cause damage to the eyes.

Application of risk control measures

The application of risk control measures falls broadly into four areas:

• Hardware

• System

• Procedures

• People

Hardware

Switchgear and electrical panel construction

Design plays a key role in the mitigation of the electric shock and arcing hazards.

In the case of electric shock, the provision of a single isolator to de-energise all sources of potential with appropriate means for tagging, lock out and proving is essential.

To facilitate fault finding on complex control systems which on occasion must be performed with the system energised, it is essential to segregate high energy components capable of presenting an arc flash hazard from lower hazard control components that may require frequent access.

All conductors and components should be shrouded to prevent inadvertent contact. A popular concept is ‘finger proof’ to a degree of protection IP2x.

The arcing hazard can be mitigated at the design stage by specifying appropriate arc containment and venting arrangements.

Remote operation is increasingly being adopted as a means of removing the worker from the hazard.

Existing switchgear and panels

Existing switchgear and panels can often be retrofitted with improved segregation, shrouding and shielding to prevent inadvertent contact with energised parts.

The severity of the arc hazard can often be reduced by shorter electrical protection clearance times without impairing the functionality of the system.

Labelling

Appropriate hazard warning labels should be fitted to all switchgear and panels giving panel reference number, operating voltage(s) and calculated arc incident energy and arc hazard boundary.

Systems and procedures

Written procedures

A written safe system of work must be provided. Well established standards provide guidance on this.

Permit to work

A written permit to work system is necessary to specifically authorise particular tasks to be undertaken. An essential element of this permit system is a live work permit which will usually be issued only in exceptional circumstances where no realistic alternative exists.

Risk assessments

Risk assessments are an essential part of the management of hazards and may be prepared on a generic basis for all routine tasks

Job safety assessments

A job safety assessment is normally linked to a specific task and is performed by the operatives who will undertake the work to identify any specific hazards which may arise.

Electrical isolation

Well established codes of practise place electrical isolation and the establishment of an electrically safe work condition at the top of the hierarchy of risk controls.

Work on energised electrical systems should only take place under exceptional circumstances with a written permit to work, well defined method statement and under close supervision.

A de-energised electrical system no longer presents any shock or arcing hazard to electrical workers.

A key element of any system of safe working is an isolation procedure. A typical process from NFPA 70 (E) is quoted below, verbatim.

1. “Determine all possible sources of electrical supply to the specific equipment. Check applicable up-to-date drawings, diagrams and identification tags. 2. After properly interrupting the load current, open the disconnecting device(s) for each source. 3. Wherever possible, visually verify that all blades of the disconnecting devices are fully open or that drawout-type circuit breakers are withdrawn to the fully disconnected position. 4. Apply lockout/tag out devices in accordance with documentation and published policy. 5. Use an adequately rated voltage detector to test each phase conductor or circuit part to verify they are de-energised. Test each phase conductor or phase part both phase to phase and phase to ground. Before and after each test determine that the voltage detector is operating satisfactorily.”

Live working

Live or energised working should only be undertaken when there is no alternative, and the work should only be undertaken under the authorisation of a written permit issued by a competent person.

A typical permit for live working is given below.

People

The part played by electrical workers in any system of safe working is vital and some of the factors that need to be considered are discussed below.

The importance of human factors

All systems must be developed with human factors taken into consideration. Ensuring that the interactions between equipment, procedures and their users are optimal is important.

Awareness of the hazard

Ensuring that electrical workers are aware of the hazards that they face is a key part of any electrical safety management system.

In particular, recognition of the arc flash hazard has occurred only relatively recently and many electrical workers – especially outside of the US – are unaware of the hazard.

Training and competence assessment

Every employee must be trained for the work that they are expected to undertake and their competence must be assessed.

The competence assessment process usually involves compiling a schedule of tasks undertaken together with the competencies required to safely complete the tasks.

Electrical duty holder/champion

A suitably competent person is required to ensure that any safe system of work is implemented fully and well. Any such appointment should be made formally in writing.

PPE for electrical workers

Having been through the process described above we can consider appropriate PPE for electrical workers based on residual risk after all mitigation measures are in place.

Shock hazard

Apart from some specialist operations such as live working at high voltage using conductive suits, PPE to specifically protect against the shock hazard usually comprises of insulating gauntlet style gloves.

Insulating gloves are specified by working voltage and manufactured to widely accepted standards.

Working voltage ratings are available from 500 to 36,000 volts with test voltages significantly higher.

Leather over gloves are worn with insulating rubber gloves to provide mechanical protection to the rubber insulating glove and also provide thermal protection against arcing.

Cotton under gloves may also be used to improve comfort levels.

The wearer of a glove combination will experience a significant loss of dexterity, which will impede or prevent many manual work operations; this should be taken into account when preparing task specifications.

It is common for workers routinely using insulating rubber gloves to be issued with four sets – one for use, one set of spares immediately available, one set in storage and one set away for test and certification.

An inflation test is commonly used to check for holes and damage in rubber insulating gloves.

Some codes 4 call for a shock hazard analysis and define various approach zones to energised conductors.

Arcing hazard

An electric arc can release devastating amounts of energy as well as many other undesirable products. The current state of the art is to protect against the thermal effects of an electric arc.

To correctly specify PPE it is necessary to complete arc flash hazard calculations to determine the incident energy at the working distance, which enables us to specify the correct Arc Thermal Protective Value (ATPV) of the PPE.

The full ensemble necessary to give protection against the thermal effects of arcing comprises:

• Body protection – Manufactured and appropriately certified to the necessary ATPV value, may be single layer or multi layer to suit particular requirements. Appropriate cleaning and repair processes are vital to maintain thermal protective properties

• Hand Protection – Gloves with an appropriate ATPV value are required. It should be noted that since incident energy is approximately proportional to the inverse square of the distance from the arc. If you halve the distance you increase the incident energy by a factor of approximately four. This means that gloves with a significantly higher APTV than the value calculated for the torso are required

• Head protection – Head protection is usually afforded by an appropriate combination of hard hat, visor/safety glasses and balaclava

• Footwear – As of October 2012 there were no arc rating standards for footwear; however, heavy duty leather industrial safety boots are usually considered to be adequate

Summary

Protective workwear for electrical workers can only be correctly specified when the residual risk, after all hazard mitigation measures have been implemented, has been identified and quantified.

In order to correctly specify PPE for the arcing hazard it is necessary to complete an arc hazard study in order to determine the incident energy at the working distance, which enables us to determine the correct ATPV for workwear.

Arc flash hazard calculations determine in the severity of exposure in order to specify the correct protective value for workwear.

Risk assessments determine when deployment of PPE is necessary.

References 1. Health and safety: risk management. Dr Tony Boyle. ISBN 978 0 901357 41 0 2. UK Health and Safety Website www.hse.gov.uk/toolbox/electrical.htm 3. Daniel Roberts IEEE Paper 'Risk Management of Electrical Hazards' ESW2012—17 4. NFPA 70E 2012 Handbook for Electrical Safety in the Workplace pp48 5. UK Heath and Safety Website www.hse.gov.uk/electricity/injuries.htm 6. UK Health and Safety Publication HSG 85 'Electricity at Work Safe Working Practices' pp5 7. BS EN 60529:1992 (IEC 60529:1989 second edition) Specification for degrees of protection provided by enclosures (IP code) 8. US Department of Energy - Brookhaven laboratory www.sbms.bnl.gov/sbmsearch/subjarea/192/192_exh4.cfm 9. Helicopter and High Voltage Power Line Maintenance www.youtube.com/watch?v=4SX6Ucbb1l8 10. IEC EN 60903:2003 Live working. Gloves of insulating material 11. ASMT D120 Standard Specification for Rubber Insulating Gloves. 12. IEEE 1584 Guide for performing Arc Flash Hazard Calculations 13. Electrical Construction and Maintenance www.ecmweb.com/content/prepare-your-plant-arc-flash-hazard-study

Published: 18th Oct 2012 in Health and Safety Middle East

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