After going through this chapter the trainee will be able to tell about
1) The necessity & awareness of safety in handling electrical equipment
2) Station grounding
3) Do’s & Don’ts on electrical equipment,
4) Comprehend and put to use the basic safe practices that are to be followed in day-to-day work.
Safety and grounding is the most important aspect of any electric job. Every one who works on or around electrical equipment must know what the dangers are and what steps can be taken to make sure that all personnel’s are protected from these dangers.
1.1 ABOUT CURRENT & VOLTAGE:
Whenever a person comes in contact with an energized conductor, the current will pass through the person’s body. For example, if a person is touching energized conductors with both hands, or if he is touching an energized conductor with one hand and a grounding object with the other, the current will pass directly through the person’s heart. However, if only one hand is touching an energized conductor, the current travels to grounding along the path of least resistance. In this case, the current moves up the arm, down the body, and out through the nearest leg. The current tends to stay on one side of the body, so it is less likely to pass through the heart.
The body is a conductor, and the severity of electrical shock depends on how much current passes through the body, the path that it takes and the physical dimensions of the human.
The human body offers resistance (1,00,000-6,00,000 ohms in dry condition and 10,000 ohms in case of wet condition) to the passage of current through it.
Different people have different amount of resistance, and different parts of the same body have different resistance values. Resistance can also vary from one minute to the next. Resistance is affected by factors such as individual physiology, a person’s emotional state, and the moisture that may or may not be on a person’s skin.
The amount of current flow passing through a body depends on three factors.
- The voltage of the source
The amount of current flow passing through a body depends on three factors.
(1)The voltage of the source
(2) Body resistance along the current path.
(3) Current capacity of the source.
The current level is greatest when voltage is high and resistance is low. Touching an energized spark wire having voltage of 30000 V will result in a shock but not a dangerous a 30 V battery with a high current capacity can result in death if body resistance is low enough. In general any current source at 30 V or more must be considered dangerous.
1.1.1 CURRENT AND It’s EFFECTS:
Sr. Current Physiological effects Description No magnitude
|1||1 mA||Threshold of perception||A current at which a person is just able to detect a slight tingling sensation in his hand or finger tips|
|2||1-6 mA||Unpleasant to sustain||This is often termed as let go currents. Do not impair the ability of a person holding an energized object to control his muscles and release it.|
|3||6-9 mA||Threshold of muscular||These are threshold values, since 10.5 mA contraction current & 16 mA current are the let go values for women & men respectively|
|4||9-25 mA||Muscular contraction||May be painful and can make it hard or impossible to release energized objects grasped by the hand|
|5||25–60 mA||do||Make breathing difficult|
|6||60-100 mA||Ventricular fibrillation||Ventricular fibrillation, stoppage of heart or inhibition of respiration might occur and cause injury or death if time is more than 1 sec.|
1.1.2 AIDING ELECTRICAL SHOCK VICTIM :
The first thing to do is to cut the Electrical Power, if possible. If the Power cannot be cut, an insulated pole or rope should be used to move the conductor away from the victim. The victim should not be touched while he is in contact with the conductor, otherwise the person touching could also become a shock victim. If the victim has stopped breathing or if his heart has stopped breathing, artificial respiration or cardio-pulmonary resuscitation (CRP) will be necessary to revive him.
Grounding is an electrical connection with the general mass of the earth through an earth electrode. The purpose of grounding is to maintain the surface under and around the station at as nearly as zero potential, so that, working staff are not exposed to the danger of electric shock. It is also meant for system grounding, so that over voltages, arcing ground etc are avoided. Thus, during earth faults current flows through the least resistance path i.e. ground connection and not through human body and thereby ensuring safety
2.1.1 SYSTEM GROUNDING:
Grounding of electrical neutral point of equipment is called system grounding. For example, Generator neutral, Transformer neutral, etc.
The purpose of system grounding is for the following reasons;
(1) Earth fault detection is possible by providing CTs, relays and associated circuits.
(2) Rise of system voltage of healthy phases during earth fault is restricted, thereby avoiding damage to equipment.
(3) Arcing grounds and consequent system disturbances & equipment damages are avoided.
2.1.2 EQUIPMENT GROUNDING:
Grounding of non-current carrying metallic parts is called equipment grounding. The following are examples;
(1) Motor body, switch gear metal enclosure, transformer tank etc.
(2) Support structures, towers, poles, cable tray etc.
(3) Body of portable equipment such as iron, oven etc.
As seen from table in section 1.1.1, by grounding electrical equipment, shock currents can be kept below the critical damage current.
2.2 SAFE PRACTICES:
Below mentioned are the safe practices to be followed when handling any electrical equipment or during routine checks.
(1) An earthing switch Switch yard is to be operated only when a valid OTO exists and the relevant circuit is shutdown from both ends: in case of line feeders and from our grid, in case of Transformer feeders & Main Bus.
(2) When manual operation of earth switch is found to be hard, immediately stop further operation and check whether associated isolators are closed. Inform maintenance section for necessary action / help.
(3) Do not isolate / operate any equipment without authorized OTO and approved procedure.
(4) Always maintain required minimum clearance when performing routine checks or any vehicle / material movement in the conventional part of the switch yard.
Description Minimum clearance (mm) 400 KV 220 KV
(i) Phase to ground 3400 2100
(ii) Phase to phase 4200 2100
|Description||Minimum clearance (mm)|
|400 kv||220 kv|
|(i) Phase to ground||3400||2100|
|(ii) Phase to phase||4200||2100|
(5) Do not carry long poles or grounding sticks / in vertical position when moving in the conventional part of switch yard.
(6) While providing temporary earthing using earthing tools, first connect the earthing cable to ground and then to the equipment to be earthed.
(7) While normalizing any temporary earthing, first disconnect at the equipment side and then on the ground side.
(8) Cleanliness of the areas is always to be ensured. No oil spills / grease etc that may cause injury to the working personnel should lie on the floor.
(9) The rooms housing the electrical equipment should be dust free. For this, working of ventilation system both exhausts & supply fan should be ensured during routine checks.
(10) Mopping of areas inside electrical switch gear rooms should be under strict supervision by an authorized person. In no case panels shall be touched by moping sticks.
(11) Before performing any MCC cell/ SWGR isolation, check availability of following
a) Work permit
b) Authorized OTO
c) Connected load is off & tagged in C/R.
(12) Do not switch off a load with cell isolator as the isolator is not intended for breaking current. Any attempt to do so will cause injury to person as well as damage the equipment.
(13) Do not try to force open the MCC cell / SGR door. If isolation problem exists, then consult the electrical technician for help.
(14) Before normalizing any MCC cell / SWGR ensure the following;
a) Work permit is signed & surrendered for normalization.
b) Authorized normalization OTO exists.
c) Connected load is off.
d) Cross check the particulars on the OTO & field for correct cell normalization.
(15) For installation of fuses in the fuse holder, ensure that correct fuse puller is used. Never try to install fuses by hands or other means.
(16) Always ensure ventilation fans are in running condition in electrical equipment rooms; especially in Battery rooms.