Various regulations and guidance documents are applicable to the UK including:

Electricity At Work Regulations (E.A.W.R.) 1989
This covers electrical systems, equipment and conductors and work activities on or near electrical equipment.

The purpose of the Regulations is to require precautions to be taken against the risk of death or personal injury from electricity during work activities.

Regulation 14 – No person shall be engaged in any work activity on or so near any live conductor* that danger may arise unless:-
a) it is unreasonable in all the circumstances for it to be dead, AND
b) it is reasonable in all the circumstances for him to be at work on or near it while it is live, AND
c) suitable precautions (including where necessary the provision of suitable protective equipment) are taken to prevent injury.

* - other than one covered with insulating material so as to prevent danger

When looking at Regulation 14 companies MUST consider other means of achieving safety prior to consideration of use of PPE, e.g. elimination of hazard, engineering controls, safe systems of work

http://www.opsi.gov.uk/si/si1989/Uksi_19890635_en_1.htm

HSR25 – Memorandum of Guidance on the Electricity at Work Regulations 1989

This is intended to help duty holders meet the requirements of the Electricity at Work regulations 1989.

It sets out the Regulations and gives technical and legal guidance on them.  (Not applicable to mines and quarries, separate guidance is available for this). The purpose is to highlight the nature of the precautions in general terms to help in the achievement of high standards of electrical safety in compliance with the duties imposed.
http://www.hse.gov.uk/pubns/books/hsr25.htm

HSG85 Electricity at Work – Safe Working Practices

This provides guidance when devising safe working practices for people who carry out work on or near electrical equipment. It covers:

1. Hazards;
2. Equipment;
3. Assessing safe working practices;
4. Deciding whether to work dead or live;
5. Actions common to both dead and live working

You can buy it here

HSG230 – Keeping Electrical Switchgear Safe

This is intended to assist managers, engineers and other relevant personnel to understand their responsibilities and duties in the operation, care and maintenance of high-voltage and low-voltage switchgear, with a view to keeping it safe. There is guidance on the selection, use, care and maintenance

A concise guide is available:
www.hse.gov.uk/pubns/indg372.pdf

The full document can be purchased from HSE here.