No person must undertake any work on or near any part of a high voltage system unless that part of the system is:
- Isolated and locked off from all points of supply (including voltage and auxiliary transformers, common neutral earthing equipment and other sources from which the apparatus and conductors may become live) and caution notices fixed at all points of isolation;
- Proven not to live and discharged;
- Connected to the earth, by approved means, at all points of disconnection of high voltage supply from the system;
- Screened where necessary to prevent danger and danger notices attached to or adjacent to live apparatus at the limits of the zone on which work may be carried out;
- Released for work by the issue of an appropriate safety document.
It is the duty of the person issuing the appropriate safety document to ensure compliance with the above in the correct sequence and in accordance with an agreed switching schedule.
- ISOLATION OF EQUIPMENT AND CONDUCTORS
Isolation or re-connection of high voltage equipment or conductors, for the purposes of carrying out work, must not be initiated without the authority of the Supply Leader.
Exclusive safety locks must be used to (a) lock closed all shutters or doors protecting live conductors and (b) to prevent an earthed circuit from being energized from any point. Safety locks must not be removed until the relevant Safety Document has been cancelled.
Where applicable, operational locks may be used for all locking-off procedures which do not require safety locks such as off-load tap changer switches on transformers.
- EARTHING OF HIGH VOLTAGE EQUIPMENT
Absent of Voltage
When high voltage equipment and conductors are to be discharged and earthed, the equipment should first be checked for the absence of voltage by means of an approved high voltage testing device in accordance with the procedure described in Appendix – Safety Precautions for the Testing of High Voltage Electrical Equipment. (TO BE ADDED) The high voltage testing device must itself be checked immediately before and after the verification.
Earthing of the high voltage equipment and conductors shall then be carried out:
Where practicable by the use of a circuit breaker or earthing switch provided for the purpose to make the earthing connection. When the circuit breaker is used, the trip feature shall normally be rendered inoperative before closing, unless this is not practicable. After closing, the circuit breaker or earthing switch shall be locked in the earth position, so that it may not be opened whilst it forms the circuit main earth.
Where this is not practicable or not applicable, the high voltage equipment must be discharged and earthed by earthing leads).
Additional earth may be applied at or near the points of work in accordance with an approved procedure after the issue of a Permit to Work. Such additional earth may be attached or removed by a competent person.
Procedure for the Use of Earthing Leads
Earthing leads associated clamps and earth application devices shall be of an approved type and of adequate capacity for the duty at the point of application. All items used for earthing shall be adequately maintained and always examined immediately prior to use.
The procedure to be followed when using earthing leads shall be:
Verify that the circuit is not live and test by means of a high voltage indicator of an approved type, the indicator itself being tested immediately before and after the verification
- Discharge all three phases using the approved earthing device with the earth lead clamped to the earth bar. (The authorized contractor, standing back as far as possible, shall discharge all three phases to the earth by means of the approved earthing device.)
- Earthing leads shall then be connected to the earth system before being secured to the phases. Care must be taken to ensure that good contact is made.
- All phases shall be earthed, even if work is to be carried out on one phase only.
- Earthing leads shall not be applied in any cell or compartment in which there is any exposed live metal at high voltage.
- When earthing leads are being removed, they shall be disconnected from the phases first and the earth system last.
- SAFETY DOCUMENTS
Corporate form HW PTW – Electricity – High Voltage System is the safety document governing all high voltage work at this site.
The different safety documents and their related procedures are described below:
- Permit to Work – HW PTW
- Authority for Issue
- A Permit to Work must be issued by the Competent Person before any work is carried out on equipment or conductors.
- Procedure for Issue and Receipt
All Permits shall be written legibly. (The use of block capital letters is preferred.)
- A Permit to Work shall be explained and issued to the Qualified Contractors who is to carry out the work. The Competent Person shall then read back the contents to the person issuing the permit, confirming that he understands it and is conversant with the nature, extent, and limitations of the work to be done. The Competent Person shall then sign the Permit and its duplicate copy.
- The recipient of a Permit to Work shall retain the Permit to Work in his possession at all times whilst work is being carried out.
- Where more than one Working Party has involved a Permit to Work shall be issued to each Competent Person carrying out a section of the work. Each Permit to Work shall be cross-referenced one with another.
- When work is to be carried out by a third party, then the following shall apply:
- Either (i) a Permit to Work shall be issued directly to the third party Competent Person who is to carry out the work. This should take place in the presence of the third party Supervisor or another nominated person in charge of the work to be carried out unless otherwise agreed in a written contract.
- Procedure for Clearance and Cancellation
A Permit to Work shall be cleared and cancelled;
- When work on the apparatus or conductor for which it was issued has been completed..
- When it is necessary to change a person in charge of the work detailed on the Permit to Work.
- At the discretion of the Supply Leader when it is necessary to interrupt or suspend the work detailed on the Permit to Work.
The recipient shall sign the Clearance section and then return to the Competent Person or his deputy who will cancel the Permit to Work by signing the Cancellation section. In all cases, the recipient shall indicate in the clearance section whether the work is “complete” or “incomplete” and that all gear, tools and additional earth “have” or “have not” been removed.
Where more than one Permit to Work has been issued for work on High Voltage equipment or conductors associated with the same circuit main earth, the Senior Authorised person shall ensure that all such Permits to Work have been cancelled before the circuit main earth are removed.
At no time does anyone leave the site with a Safety Document in his possession. The holder of the Safety Document must hand it to the competent person or a nominated person, before leaving the site and must not resume work before a valid permit is in his possession.
Feeder and Voltage Transformer
When work is to be carried out on feeder and voltage transformer spouts or on the busbar spouts of a single panel, the following operations shall be carried out:
- The spouts on which work is to be carried out shall be isolated from all points of supply from which they can be made live. Attention is drawn to the need for the isolation of any voltage transformer associated with the relevant circuits.
- It is essential that both primary and secondary sides of voltage transformers are electrically isolated when voltage transformers have to be isolated. The isolation of voltage transformers should preferably be carried out with the voltage transformer de-energized.
- The isolating arrangements shall be locked so that they cannot be operated and the shutters of live spouts shall be locked shut (this includes the voltage transformer spouts where fitted). The shutters of all other spout not immediately to be worked on shall be locked shut.
- The spout contacts shall be checked by means of an approved voltage indicator to verify that they are not live, the indicator itself being tested before and after the verification.
- A circuit main earth with approved earthing equipment shall be applied at the point of work and were practicable at all points of isolation from the supply.
- Caution notices shall be attached at all points of isolation of the circuit.
- HIGH VOLTAGE CABLES
- No work shall be carried out on a high voltage cable unless the cable is dead and earthed. Attention is drawn to the need for the isolation of any voltage transformers associated with the cable.
- When work is to be carried out on a high voltage cable the Competent Electrical Person or designate, after isolating and earthing the circuit concerned, shall positively identify the cable at the point of work by suitable means such as visual inspection or tracing equipment.
- Where necessary, the Competent Electrical Person or designate shall then spike the cable at the point of work using an approved remotely operated spiking gun.
- WORK ON TRANSFORMERS, REACTORS AND CAPACITORS
- When work is to be carried out on the connections to the windings of transformers, reactors and capacitors all sources of supply, including possible back feeds, shall be isolated and locked off.
- A discharge time, in accordance with the manufacturers’ recommendation, must be allowed for capacitors before further work is carried out. Where capacitors are provided with discharge shorting or earthing switches, they shall be used in accordance with the manufacturers’ instructions.
- The transformer or reactor or capacitor shall be earthed with approved earthing equipment at the point of isolation from the high voltage supply.
- Caution notices shall be fixed to all points of isolation including those on the low voltage system.
- WORK ON ROTATING ELECTRICAL MACHINES
- Rotating high voltage electrical machines shall be isolated from their source of supply.
- A Circuit Main Earth shall be applied at the point of isolation before a Permit to Work is issued.
- Caution notices shall be posted at all points of isolation including the points of immobilization.
- EQUIPMENT OPERATED BY OR CONTAINING COMPRESSED AIR
- In addition to specific equipment safety requirements issued by the manufacturer, the following precautions shall be taken before any work is carried out on high voltage equipment operated by or containing compressed air:
- The valves controlling the supply of air to the high voltage equipment shall be closed and locked by safety locks.
- Air should be released in the work zone and pipe work shall be vented to the atmosphere by the use of a suitably sized vent valve which shall be padlocked open.
- Caution notices should be attached to each valve.
- Operating adjustments which require the normal air supply may be carried out but only under the direct supervision of an Authorised Person.
- ELECTRICAL PROTECTION
No electrical protective device associated with high voltage apparatus forming part of the electrical distribution system shall be adjusted, put into commission, or taken out of commission without the sanction of the Competent Electrical Person or designate
The operation of all protective gear, which is either new or has undergone substantial modification or repair, shall be proven to the satisfaction of the Competent Electrical Person or designate before the high voltage equipment is put into commission.
Before any high voltage equipment is connected to the system for the first time or re-connected after substantial maintenance or alteration, the setting of the associated protective gear shall, for the time being, be adjusted to the minimum practical value so as to give maximum protection under fault conditions. The protective devices shall be returned to normal settings immediately prior to being returned to normal service.
- SAFETY PRECAUTIONS FOR THE TESTING OF HIGH VOLTAGE EQUIPMENT
The testing of high voltage equipment may be divided into two categories:
- Voltage testing with the equipment possibly or actually live;
- Testing under the terms of a High Voltage to Work Permit.
- 1 VOLTAGE TESTING
This is normally carried out to establish that conductors are not live or to verify the phase identity of two circuits before paralleling. Voltage testing shall be carried out only by an external qualified electrician accompanied by a Competent Person. This second person shall remain at a safe distance during testing. Where there is reason to expect conductors to be live, then warning notices shall be posted at the entrances to the Substation and, where necessary, the test area taped off.
The use of a voltage testing device to establish that conductors are not live requires that the device be checked for correctness of operation in accordance with the manufacturer’s instructions before and after use. The voltage testing device shall have a valid test certificate and should be visually inspected for signs of damage before use. A damaged device must not be used. The device must be wiped with a clean lint-free dry cloth to remove any moisture prior to use.
Before use, the voltage testing device must be checked using a suitable proving unit. It must indicate the expected output of the proving unit. The earth clip of the device should then be securely connected to a bare metal-earth point and the probe applied to the conductors under test. A zero reading indicates that a conductor is not live. Where the system being tested contains conductors which are connected to different phases it is vital that all conductors are proven to be not live.
Immediately after use, the voltage testing device must again be checked for correct operation using the proving unit.
Where it is necessary to verify the phase identity of two circuits prior to paralleling it is strongly recommended that this be done by switching in transformers and carrying out the comparison at low voltage using accurate instruments. Where this is not practicable then phase comparison can be carried out at high voltage using two voltage testing devices connected together in accordance with the manufacturer’s operating instructions. The technique is only valid if the neutral earth points of the systems being compared are connected together. If this technique is used then both devices must be examined and checked as explained above.
This procedure is only for reference – always follow safety rules prescribed by your local H&S department.