Chapter (6)

Safety Precautions and Safety Procedures for maintenance of circuit Breakers

Electric ShokcsElectricity is dangerous. However, it is very useful and has become essential.

Hence everyone should know the safety precautions to be taken while working with an. electrical device or while maintaining an electrical device. Careless use of electricity is dangerous.

The electric shock of even 50 Volts a.c. can be dangerous in, the wet surroundings. Several, deaths take place, every year due to electric shocks at -230 Volts a.c.' residential buildings particularly in wet surroundings like bath room. Once, the, person, comes in good contact with a live part and is in touch with - wetground, he cannot get separated from the live part and his musceles get paralysed and he gets electrocuted.

The resistance of dry skin of human body (between tip -of left-hand finger and right hand finger) is; of the order of 100,000 ohms.

However, when the hands are wet this may drop to below 5000 ohms. Similarly the resistance of the skin of feet is high when dry and low when wet.

Hence wet surroundings are dangerous.

The affect of electric shock on human body: The affect of electric shock depends upon the voltage,., current and duration and also the path of flow of current through, the heart, the situation is most dangerous. The effects are as, follows :

Magnitude of Current EffectBelow 10 10 milli-amps10 mA to 15 mA15 mA to 20 mA20 mA to 40 mA40 mA to 80 mA

Above 100 mA

- Mild sensation, but not painful- Painful shock but Muscles still in control- Muscle control affected - Muscle contractions, Breathing affected .

- Rapid, unco-ordinated series of contractions of heart muscle causing irregular heart beat (fibrilation) and possible death.

- Severe burns, muscular contrac tions, stoppage of

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heart. Death certain.

AC voltage has more peak value than d.c. voltage of . same level hence it is more dangerous.

If the contact is of a very short duration due to throwing off of the person, the lile is saved.

The immediate action if a person has been attached to live part due to shock:

- Switch off the supply.

- Remove the person from the live part by means of insulating bars.

- Give artificial respiration to the person

- Call the doctor.

The precautions to' be taken while maintaining/testing/commissioning electrical equipment.

Safety Procedures

1. Follow the safety rules faithfully.

2. Take permission from authorised person for doing specific work.

3. Make sure to switch-off the supply, from both ends. The switching-off "I and switching-on should be as per safety rules and with prior permission of the authorised person.

The repair/maintenance work of High Voltage Apparatus should not be undertaken unless the apparatus is made DEAD and Isolators are open and locked.

4. Place caution, notices and Danger notices near' the work place and, near the switching terminals.

WATCH! DONT SWITCH-ON

MEN AT WORK

DANGER VOLTAGE

DONT TOUCH

5. Keep barriers, ropes around the section under maintenance to clearly indicate maintenance zone and boundary of the neighbouring live zone.

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6. Earth the various metallic parts of structures, bus sections, conducting parts etc. at two or more places before commencing the maintenance work.

7. Be familiar with main circuit and auxiliary supply circuits. Switch off both.

The recommended precautions to be taken before working on High Voltage Apparatus. (Above 650 V).

No person shall undertake any repairs, maintenance, cleaning, alteration or such work, on any 'part of High Voltage Apparatus unless such parts of the apparatus are Dead ;

Isolated and all particable steps taken to lock off from live ,conductors ;

Efficiently connected to earth at all points of disconnections of -supply to such apparatus, or between such points, and the point(s) of work ; Caution Notices fixed

Screened where necessary to prevent Danger and Danger Notices fixed

Released for work by the issue of a Termit-to-Work or Sanction-for- Test ;

and unless such person is fully conversant with the nature and .-also the extent of the work to be done.

It is the duty of the person issuing the Permit-to-Work or Sanction- for-Test to ensure that the foregoing provisions are ,complied with.

(a) Cleaning and painting of earthed metal enclosures, connections of circuits to or from live high voltage systems live line testing and live insulator washing may be carried out but only in accordance with the Special instructions relating to these purposes issued by the Chief Engineer of the Electricity Board concerned.

(b) Live L hie Work on high voltage overhead lines may beout in accordance "with rule.

(c) where the design of apparatus precludes the strict compliance with all details of these precautions, the work shall be carried out to the instructions of a Senior Authorised Person who must be present, and after agreement with the Control Engineer.

8. Check the Safety Clearance8 between nearest five points and other physical objects handled during maintenance (e.g. ladders, platforms9 lifting devices, metal-bars, etc.). Safety clearances must always be, maintained. Otherwise the flashovers can result.

9. Recommended precautions to be taken before working on Medium and Low Voltage systems.

Medium Voltage 250 V to 6.50 VLow Voltage below 250 V

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Precautions to be taken before working on Medium and Low Voltage Systems. The consequences of shock or serious burns from short circuit associated with medium or low voltage systems may be serious or, in some circumstances, fatal. Wherever practicable, therefore, work on Medium and low Voltage Apparatus conductors and equipment shall be done while they are, dead and earthed.

When working on dead Medium and Low Voltage Apparatus suitable precautions should be taken by screening or other means avoid danger for inadvertent contact with live conductors within the working zone.

It is not always possible to make dead or earth Medium and Low Voltage Apparatus. All work on Medium and Low Voltage Apparatus must be carried out as if it were live unless. it is proved dead.

When working on live Medium and Low VoltageApparatus suitable precautions should be taken by screening or other means to avoid danger from inadvertent contact with live conductors of earthed metalwork.

Work on live, Medium and Low Voltage Apparatus. conductors or equipment should be undertaken only by a Competent Person.

Note. Attention is. drawn to the fact that-, certain statutory requirements prohibit work on live medium and low voltage apparatus, conductors or equipment.

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Example of a typical form for electrical maintenance access to work (Permission to approach the work place)

TYPICAL Form: ACCESS-TO-WORK

THE ............. STATE ELECTRICITY BOARD

OFFICE OF SUPERINTENDING ENGINEER

O AND M DIVISION

ACCES TO WORK.

Permit No .............

To, .............................................................................................................................

(Name) (Designation)

in the employment of .................................................................................................. is hereby granted permission to carry out work described below.

Location ......................................................................................................................

Work .........................................................................................................................

......................................................................................................................................

......................................................................................................................................

......................................................................................................................................

......................................................................................................................................

........................

NO OTHER WORK SHALL BE CARRIED OUT

Remarks ..................................................................................................................... ........................................................................................................................................................................................................................................................................................

*Appropriate Control Engineer notified

Time ...................Date......................

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Signed.........................................................................................................................

being a person authorised to issue this card.

Time ...................Date......................

Example of a typical form, for Permit- to- work.

TYPICAL Form: PERMIT-TO-WORK

THE ............. STATE ELECTRICITY BOARD

OFFICE OF SUPERINTENDING ENGINEER

O AND M DIVISION

Permit No .............

To, .............................................................................................................................

(Name) (Designation)

in the employment of ..................................................................................................

I hereby declare that it is safe to work on following HV Apparatus, which is Dead, Isolated and Earthed:

......................................................................................................................................

......................................................................................................................................

............ .........................................................................................................................

......................................................................................................................................

.........................

NO OTHER APPARATUS SHOULD BE CONSIDERED SAFE

The apparatus is efficiently earthed at following points

......................................................................................................................................

......................................................................................................................................

............

Following Precautions are recommended .................................................................. ...................................................................................................................................................................................

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......................................................................................................................................

...........................................................................................................

Following Work is to be carried out:

......................................................................................................................................

......................................................................................................................................

......................................................................................................................................

..................

Scheduled Time: From.............................................. To............................................

Signed .........................................................................................................................

(Name) (Designation)

Time ...................Date......................

The Safety clearances for electrical maintenance work

Clearance to earth is defined as the shortest distance between -conductor and nearest earth point.

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Fig (6.1)

Ground Clearance of an equipment is defined as the distance between the highest earthed part of the equipment and the ground.

The live point is at a certain voltage (V) above earth. The -various points in the air between the live point and the earth at different voltages starting from V at conductor surface to 0 at -earth. This voltage distribution in air is normally not visualised by untrained persons.

Refer to Fig. (6.1) A physical object A away from conductor is subjected to only 0.08 V whereas an object is near the conductor subjected to a voltage of 0*5 V across its length.

Further if the object is of conducting material, the voltage V appears across clearance and the gap may flashover.

To avoid such possibility, a safe clearance should be maintained between the line conductors and nearest objects such as, platforms, ladders, tools, persons, physical objects to avoid flashovers.

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Fig (6.2)

When. the conductors are Live, the minimum clearances to bemaintained depend upon the voltage of conductors.

Rated voltagekV ph. to ph. rms

Minimum safe clearanceMetres Feet

>6.6 kV

>6.6 kV 11 kV

>11 kV 22 kV

>22 kV 33 kV

>33 kV 66 kV

>66 kV 132 kV

>132 kV 245 kV

>245 kV 400 kV

2.57 8.5

2.59 8.5

2.64 8.75

2.75 9

3.00 9.75

3.43 11.25

4.57 15

5.48 18

There ore instances when persons who are in their working place in the factory experience momentary electric shock in theirwork place. The investigations of such shocks reveal&, the, following.-, possibilities :

1. The shock originates in sparking between live part and neighbouring metal part such as cabinets, structures, piping work etc. due to poor insulation or reduced clearance. The sparking may be momentary.

The metal parts are continuously connected through structural frame-work, water piping, earthing matt, foundation plates.

During the spark-overs, the persons in contact with the metal structures etc. get a shock.

2. High voltage conductors are capacitively coupled with the earth. Each conductor and ground is at certain potential with respect to ground under certain conditions

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the person may touch intermediate point which is at a potential with respect to ground and get a shock. (Fig. 6.2).

3. The capacitance of apparatus (particularly capacitors, bushings, windings etc.) gets charged and remains charged even after disconnection from supply. A person touching such apparatus gets a shock. Before working on the apparatus, the parts should be earthed capacitors should be discharged.

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