Electrical Systems and Generators

State the causes of single phasing (WS p. 94)

o Fuse blown. o Contactor fault - dirty, maladjusted or worn contacts. o Broken supply cable. o Faulty starter/switch. o Loose or broken terminal connections. o Burnt out winding.

Explain why excitation is required on an AC generator

To provide DC magnetising current for the field magnet system. If the magnetizing current for the field magnet was AC, the resulting induced voltage would be too unpredictable.

State the term "slip". Explain why an induction motor requires "slip".

"Slip" is the difference between synchronous speed Ns (speed of rotation of magnetic field) and rotor speed Nr. Nr<Ns. Amount of "slip" depends on load. Greater load=greater slip. For emf to be induced in the rotor windings, there must be relative movement between the conductors and field, and so this machine can never run at synchronous speed.

State the device fitted to protect against short circuits on an induction motor. Explain its operation.

(WS p. 81) 1. Motor protection circuit breaker 2. Fuse

Reference to induction motors, explain why the circuit breaker may not protect against short circuit currents

(WS p. 81) Circuit breakers are supplied with a contactor to allow automatic control of motor startup and disconnection. If the contactor tries to open (when tripped), in case of short circuit current, then massive arcing will occur at the contacts, and the current flow may not be interrupted.

Three Phase AC Motor Fault Diagnosis/Maintenance & Stripping (WS p. 94)

(WS p. 94)

Describe the routine maintenance procedure for vessel's storage batteries. Outline the safety precautions that should be observed.

- Keep clean, dry and the electrolyte level correct (above the top of the plates). - Make sure the terminals are tight and the cables sound and undamaged. The terminals should be smeared with petroleum jelly to prevent oxides from forming on them. - The tops of the cells should be kept clean and all ventilation holes in the plugs kept clear of dirt, etc. Watch out for leaks and for loose mountings. - Avoid heavy charging and discharging of lead acid batteries. - Rigid mounting is important since shock loads resulting from sudden movements can damage the cell structure. - Avoid leaving a battery for more than a month without topping up the charge. - Keep cool and away from the engine in any event to protect casings.

State the advantages of Alkaline batteries over lead acid batteries

- Lighter in weight for the same energy content. - More robust, can withstand rapid and high charge and discharge rates. - Longer life and requires less maintenance. - Easier to store. - Alkaline batteries can be left in any state of charge or discharge. - Retains charge on open circuit and can be left discharged for long period without any adverse effect. (Lead acid batteries will discharge on open circuit and requires regular maintenance and charging to keep it fully charged. If left in a discharged condition for any length of time the battery may be ruined).

Describe the response of the voltage output of an AC generator when a sudden large increase in load occurs

- Load increase will cause a voltage drop in the generator ac windings, known as 'voltage dip'. - Load shedding will produce an over-voltage at the bus bars.

Reference to fuse protection of electrical circuits, state the advantages of a High Breaking Capacity (HBC) fuse. Explain how the advantages are achieved (WS El. p. 63)

- Very reliable and can be used where discrimination is required. - Does not deteriorate and has a high speed of operation. - Some are designed with a low melting point alloy on one of the links. This melts on a sustained overload, giving 2 levels of protection. This is a cartridge type fuse in which a silver fuse element is connected between two end contacts of a ceramic tube filled with a special quartz powder. When the fuse element blows, it will break at every one of the narrow points, producing a large gap between the end caps. The energy produced melts the quartz powder turning it into a glass fusion of high resistance material; this causes the arc to extinguish.

Induction motor operation

- winding passes through slots of stator. when 3 phase ac current passes through it, it produces a rotating magnetic field. - electricity is inducted in the rotor by magnetic induction rather than direct electric connection - rotational mechanical power is transferred through a power shaft to the load - electrical energy is entered through the stator - motor is inherently self starting

Reference to preference trips, list 3 for each: 1. Non essential circuits 2. Essential circuits 3. Emergency circuits (WS El. p. 58)

1. - air conditioning, accommodation ventilation fans - cargo refrigeration plant - air compressors, deck equipment 2. - Steering plant - Propulsion plant - Navigation equipment 3. - emergency generator - emergency lights - firefighting equipment

State the consequence of a single earth fault occurring on: 1. An insulated neutral distribution system 2. An earthed neutral system

1. Because there is no easy path back into the electrical system very little current will flow through an earth fault on one phase. Although the fault current is negligible, over voltages are high. The transient is likely to be 2.5 x line voltage. 2. Because current flow has a path through the hull steel and earthed neutral back to the system, fault current is high and the over-voltage due to an earth fault is low.

Reference to storage batteries, explain the following 1. Boost charge 2. Slow charge 3. Trickle charge 4. Float charge

1. Boost charge: A high rate of charge over a short period, usually applied to emergency batteries after a period of discharge to bring them quickly back to full capacity, on automatic charging systems usually followed by :- 2. Finishing rate: A low rate of charge used to ensure that the battery is fully charged but does not overheat or gas excessively. 3. Trickle charge: A small charge (adjusted to suit each installation) applied to the battery, which is just sufficient to compensate for internal losses. 4. Float charge: A charging system used for a UPS system where there must be no interruption of supply to the load. The charge will be sufficient to supply the load & provide a trickle charge to the battery.

Generic Procedure for Manual Paralleling AC Generators

1. Ensure the Prime Mover is in stand-by condition - check oil/fuel/water etc. 2. Run up the prime mover 3. Check the operation of the prime mover to confirm correct systems operation 4. Switch on the excitation (if appropriate) and check the incoming machine voltage 5. Check the voltage & frequency of the "in line" and incoming machine, making sure they are exactly the same. 6. Switch on the synchroscope and ensure that the "check synchroniser" is engaged 7. Adjust the speed of the prime mover, using the speeder (slow-fast) switch, until the synchroscope is indicating a slow rotation in the clockwise direction. (so that the incoming machine is going slightly faster than the ones on line) 8. When the incoming machine is exactly in phase with the busbars, the synchroscope will be passing through the 12 o'clock position. 9. Close the main breaker just before the synchroscope reaches the 12 o'clock position 10. Adjust the speeder switches of the incoming & in line machines until their share of the load is equal 11. Adjust the busbar frequency if necessary 12. Check that the power factors of each machine are equal (compare Kilowatts & Amps are equal on each machine if a power factor meter is not fitted) 13. Adjust the excitation of each machine to achieve equal power factors on each machine 14. Finally check the operation of the incoming prime mover.

Explain the effects that would be observed should an AVR give a low output when the generator is: 1. Running in parallel 2. Running on it's own

1. If Gen 1 and Gen 2 are load sharing, then: - the machine with the excitation loss (for ex. Gen 2) will continue to run, drawing its excitation from Gen 1. - Both generator currents will rise rapidly with Gen 1 becoming more lagging while Gen 2 runs with leading power factor (seen on PF meter) - A loss of excitation trip or an over-current trip should trip Gen 2, which may result in an overload to Gen 1. - If Gen 1 trips on over-current first, then this will deprive Gen 2 of excitation resulting in it tripping and causing a blackout. 2. If the generator is running on its own, then a loss of excitation will result in a collapse of the voltage output, and the generator tripping on under-voltage.

Explain why the electrolyte level falls during service of Alkaline and lead-acid batteries and the dangers

1. Lead Acid: On discharge, electric current converts active materials of positive and negative plates to normal lead sulphate and uses up sulphuric acid to manufacture lead sulphate. Water is produced in the electrolyte; this process leaves the acid weak at the end of the discharge. The positive plate changes from Lead Peroxide to Lead Sulphate. The negative plate changes from Spongy Lead to Lead Sulphate. The electrolyte becomes more dilute as water is formed at both plates (S.G. approx. 1.12 when fully discharged) 2. Alkaline: unlike the lead-acid system, the electrolyte does not noticeably change in composition between the charged and discharged states

Insulated Neutral system advantages (WS El. p. 51)

1. No supply interruption to affected equipment. 2. Warning on earth fault detection equipment. 3. Low Fire Risk due to low values of fault current on the insulated system. 4. Electric shock unlikely to be fatal due to very low (few mA) fault current

Describe the construction of Salient Pole AC alternator

1. Prime Mover rotating the rotor/field 2. Field rotation results in rotating EM flux intersecting armature, producing alternating EM Field 3. Magnetic Poles projecting from shaft hub and energized by: 4. Multi-turn Rotor Coils of insulated wire excited by: 5. DC Power Source which can be external or fitted on same prime mover. 6. Stator Core is built up of thin laminations, insulated from one another and divided into packet sections with a number of ventilation ducts to obtain efficient cooling. It is assembled under pressure and retained by keyed end plates. 7. 3 Stator Coils, 120 degrees phase difference from each other, star connected on one end, 3 phase drawn from other end 8. Stator Frame may be either of cast iron or of fabricated mild steel construction. f=Pn/120 P = number of pairs of poles f = frequency in Hertz n = revs/m

Insulated Neutral System disadvantages

1. Second earth fault on another phase results in a short circuit 2. Voltage from other 2 phases to earth=+73% causing possible increased stress on insulation, maybe resulting in further failures. 3. Earth fault tracing: Necessary to trip individual circuits to find affected area. 4. Over voltage: Possible voltages to earth at 3.5/4.5 x nominal due to resonance or intermittent fault. 5. Effect on other machinery: Fault currents traveling through machinery may have adverse effects - for example in bearings.

State 5 reasons that may cause the level of charge to be reduced

1. Short circuit in a cell in battery 2. Weak electrolyte 3. Loss of active material from plates (accumulates as debris at bottom of case) 4. Loss of electrolyte (or low level) 5. Extreme temperature 6. Old age

Reference to Batteries, Explain how the level of charge can be determined in each: 1. Lead Acid 2. Alkaline

1. Specific Gravity Test depends on measuring changes in the weight of the active chemicals. As the battery discharges the active electrolyte, sulphuric acid, is consumed and the concentration of the sulphuric acid in water is reduced. This in turn reduces the specific gravity of the solution in direct proportion to the state of charge. - using a hydrometer take an electrolyte sample from each cell - record the readings and compare the gravity with specific gravity of 1.28 (battery fully charged) and 1.12 (battery fully discharged) - return electrolyte to cell - Contemporary electronic sensors, which provide a digital measurement of the SG of the electrolyte, can be incorporated directly into the cells to give a continuous reading of the battery condition. 2.

Describe the construction of Induction AC motor

1. Stator - frame: provides mechanical protection and support for windings; made of steel or cast iron - varnish insulated electrical windings made of copper wire - fitted into insulated slotted laminations made of high grade alloy steel to reduce eddy current effect 2. Rotor (squirrel cage) - Cylindrical laminated core with parallel slots for carrying - Conductors: heavy bars of copper or aluminum in a squirrel cage arrangement, shortened on both sides by - End caps 3. Shaft is placed inside the rotor, so it rotates with the rotor 4. Bearings (ball or roller) for shaft support and reducing friction 5. Fan made of light aluminum, mounted on rotor shaft provides adequate cooling 6. Terminal box for holding stator and rotor windings 7. Enclosure - motor frame - end bells (caps) with bearings

Describe the procedure that should be followed should the acid come into contact with each of the following 1. Surface in a work area 2. Skin or eyes

1. Sulphuric acid splashes can be washed with a saline solution, two (2) teaspoonfuls of household salt to one (1) pint of water. 2. Contact with the body and eyes is dangerous and will cause burns. - Rapidly wash eyes and skin with plenty of water. - Skin burns should be treated with boracic powder. - Eyes should be washed out thoroughly with water followed immediately with a solution of boracic powder, one (1) teaspoonful of powder to one (1) pint of water. First aid equipment should be located locally in the battery compartment. Any accidents should be reported and logged.

Reference to shore power, State 3 necessary checks before connecting to the vessels distribution system (WS El. p. 57)

1. Supply frequency: - The operation of a 50 Hz system from a 60 Hz supply is not recommended. The motors will run faster, produce more torque, and in doing so will demand more than their rated current. This could lead to the motor being overloaded. 2. Supply phase: The phase sequence indicator will be marked to show which sequence the ship has. If the shore power is different, then swapping any two cables round will change it. 3. An inspection of the cables for damage and the connections for suitability. 4. Ensure shore supply box is off

State 5 devices fitted to a main switchboard in order to protect the AC generator

1. Under-voltage relay trips the generator breaker when a severe voltage dip (around 50%) occurs. 2. Over-current trip: the breaker is arranged to be tripped instantly only in the event of high overcurrent such as would occur with a short circuit fault. 3. Overload relay 4. Reverse Power Relay releases the breaker and prevents motoring if a reversal of power occurs. Such a device would prevent damage to a prime mover which had shut down automatically due to a fault (e.g. loss of oil pressure) 5. Preference trip

State 5 safety devices fitted to a main distribution panel

1. Under-voltage relay trips the generator breaker when a severe voltage dip (around 50%) occurs. 2. Over-current trip: the breaker is arranged to be tripped instantly only in the event of high overcurrent such as would occur with a short circuit fault. 3. Overload relay 4. Reverse Power Relay releases the breaker and prevents motoring if a reversal of power occurs. Such a device would prevent damage to a prime mover which had shut down automatically due to a fault (e.g. loss of oil pressure) 5. Preference trip

State 5 safety features of a battery locker, explaining why each is required

1. Ventilation: to prevent accumulation of dangerous gasses 2. Protective glass on lighting fittings: to prevent sparks and explosion of dangerous gasses 3. Fire detection equipment 4. Fire fighting equipment 5. Waterproofing

Lead acid batteries: state the precautions necessary when preparing electrolyte from concentrated acid

1. When the electrolyte is being prepared, the concentrated sulphuric add should be added SLOWLY to the water IF WATER IS ADDED TO THE ACID THE HEAT GENERATED MAY CAUSE AN EXPLOSION OF STEAM, SPLATTERING ACID OVER THE PERSON HANDLING IT 2. Goggles, rubber gloves and protective apron should be worn when acid is handled. 3. To neutralise acid on skin or clothes, copious quantities of clean fresh water should be available for use. 4. An eyewash bottle should be to hand in the compartment for immediate use on the eyes in case of accident. This bottle should be clearly distinguishable by touch from acid or other containers, so that it may be easily located by a person who is temporarily blinded. 5. The corrosion products that form round the terminals of batteries are injurious to skin or eyes. They should be removed by brushing, in a direction away from the body Terminals should be protected with petroleum jelly. 6. An excessive charging rate causes acid mist to be carried out of the vents onto adjacent surfaces. This should be cleaned with diluted ammonia water or soda solution, and affected areas then dried.

Incorrect paralleling will cause

A difference in voltage phase angle between the running and incoming machines at the instant of paralleling will produce a Synchronizing Current. The resulting magnetic force will try to pull the generators into synchronism, known as torque reversal. This will result in rapid acceleration of one rotor and deceleration of the other. - Breakers will trip - Blackout - Damage to machinery - Danger to personnel and vessel

Reference to AC generators, explain why they must be synchronized before connecting in parallel

A difference in voltage phase angle between the running and incoming machines at the instant of paralleling will produce a Synchronizing Current. The resulting magnetic force will try to pull the generators into synchronism, known as torque reversal. This will result in rapid acceleration of one rotor and deceleration of the other. - Breakers will trip - Blackout - Damage to machinery - Danger to personnel and vessel

State 5 causes of earth faults on an electric motor

A ground fault occurs when electricity travels through ground, instead of the intended path back to its source. 1. Deteriorated insulation on wires and cables 2. System not properly grounded

Describe short circuit and its causes in reference to electric motors:

A short circuit is a direct contact between two points of different electric potential: - Alternating current: phase-to-phase contact, phase-to-neutral contact, phase-to-ground contact or contact between windings in a phase, - Direct current: contact between two poles or between the ground and the pole insulated from it

Describe the term "insulated system"

A three phase, three wire system with no return wire. If an earth fault occurs then there is no easy path for the leakage current back to the electrical system. Three phase motors take current equally from the phases. Some imbalance is acceptable provided the load is connected in delta. Supplies for lighting, single-phase motors and other loads is taken through delta-star or delta-delta transformers.

Sketch an AVR and explain how it works. State the purpose of an AVR. (WS El. p. 20)

A) 1. Voltage Measurement element - transforms and rectifies generator terminal voltage to suitable level. - produces a low voltage DC signal that is proportional to the actual generator voltage. 2. Comparator - compares DC signal to stable Voltage Reference signal with a set DC value - Difference between the two voltages (error signal) is fed to Amplifier. 3. Amplifier output applies pulses of controlled phase to fire a Thyristor circuit 4. Thyristor adjusts the excitation level in either the generator or the exciter field. 5. Generator voltage is restored to its correct level and the 'error voltage' returns to near zero. B) An AVR senses the generator output voltage and acts to alter the field current to maintain the voltage at its set value.

Explain the term 150 Ah

Ah is the unit used to describe the quantity of electrical energy a battery could supply. 150 Ah refers to the number of hours for which a battery can maintain a given discharge current. At the 10 hr rate this battery can maintain 15 A discharge current.

Explain the operation of an inverter stating why it maybe required

An inverter is an electronic device that changes direct current DC to alternating current AC. The sine wave required is produced by "chopping" the DC voltage using 2 pairs of transistors to form the positive and negative half cycle. Method is called Pulse Width Modulation(WS p. 164) It is used to provide power to AC emergency equipment.

Describe the procedure for insulation testing a 3 phase motor

Before measuring the insulation resistance of an AC alternator, all rotating rectifiers, the AVR unit, etc. must first be disconnected. Insulation testing of Alternator and Motor windings should only be undertaken by competent persons. The system should be made safe to work on and isolated with an appropriate Permit to Work. A test voltage higher than the normal operating voltage should be used, typically 500V is used on 380 and 450V systems. The resistance between each Phase and Earth should exceed 1M OHM. The resistance of the earth lead on motors and generators should also be routinely tested with an OHM meter and should not exceed 0.1 OHMS.

State the possible consequences of incorrect synchronizing

Blackout, Likely damage to machines, Danger to personnel and vessel

State what determines that a circuit can not be connected to the preference trip

Essential and emergency circuits are not being connected to preference trips.

Explain why excitation is DC current on a AC generator

Excitation in an alternator is the current applied to the rotor to make it a magnet with FIXED POLARITY. As the rotor turns, the magnetic field is applied successively to the stator windings, causing a positive, then zero, then negative then zero etc. magnetic flux in the stator windings. The changing flux produces the generated voltage in the stator windings and the FLUX REVERSALS are due only to the rotation of the rotor. If we apply AC EXCITATION voltage to the ROTOR, then its MAGNETIC FIELD WILL ALTERNATE DUE TO EXCITATION and the STATOR WINDINGS will receive ALTERNATING FLUX VARYING both with ROTATION and AC EXCITATION. The resulting generated voltage is hard to predict.

Explain the operation of a 2 stage preference trip

If a generator overload condition develops, then the preference trip will operate to energize the timer relay. The timing relay will then time out to disconnect nonessential services in a definite order at timed intervals: - Stage 1: 5 secs - air' conditioning, accommodation ventilation fans - Stage 2: 10 secs - cargo refrigeration plant

Reference to a main distribution switchboards being fitted with preference trips, state why the preference trip is fitted

If a generator overload develops, the preference trip relay sets an alarm and acts to trip selected non-essential loads. This reduces the generator load so that it may continue to supply essential loads.

Explain the rotational speed required for a 4 pole rotor to supply a frequency of 60Hz

If f=Pn/120, 60=4n/120 4n=120x60 4n=7200 n=7200/4 n=1800 rpm

Why an insulated neutral is preferred to an earth neutral system? (WS El. p. 49 for sketches)

Insulated Neutral: if an earth fault occurs then there is no easy path for the leakage current back to the electrical system. Because there is no easy path back into the electrical system very little current will flow through an earth fault on one phase. Earthed Neutral: This is a connection of the system, via the neutral point of the alternator, to the hull steel. In the event of an earth fault the current flow has a path through the hull steel and earthed neutral back to the system. This will result in higher fault current flow than if the neutral were insulated. It is therefore essential in this instance that any equipment developing an earth fault is immediately disconnected from the supply.

State 3 factors that determine the rotating speed of a single phase motor

Nr < Ns Ns=120f/P Nr - rotating speed Ns - synchronous speed f - supply voltage frequency P - number of poles

List the types of damage due to incorrect synchronizing

Out-of-Phase Paralleling may result in: - deformation of stator windings, - movement between stator core and frame, - failure of rotor diodes (on brushless machines), - twisting of rotor shaft, - broken frames, - localized crushing of shaft end keyway and crack propagation through shaft section, - broken couplings, - marking/fracture of gear teeth.

State the possible consequences of connecting a motor in star instead of delta

Phase voltage will be lower, current values will increase, which will cause windings to overheat resulting in physical damage to motor. Single phasing?

Sketch a connection diagram for synchronizing lamps stating the lamp conditions when synchronizing should be attempted (WS El. p. 25)

Sequence Method: - Lamps are connected between the incoming generator and the busbars - One of the lamps, known as the key lamp, is connected in one phase, but the other two lamps are cross-connected. - If the two frequencies are different, the lights will light up in rotation, clockwise if running too fast, or anti-clockwise if running too slow. - The incoming machine speed is adjusted to give a very slow clockwise rotation of lamp brightness as when using the synchroscope. - The moment to synchronise is when the key lamp (which should be at the top of the triangle) is dark, and the two bottom lamps are equally bright

Describe 1 method that maybe used on a main switch board to detect an earth fault

The earth fault indicators may be either a set of lamps or an instrument calibrated in kΩ to show the IR value to earth. Two lamps are used, each rated for the full system voltage, but connected in series across the busbars with the mid point earthed. If the system is correctly insulated then both lamps will glow at half brilliance. A full earth on one pole will short circuit its lamp, causing the other to shine brightly. A slight insulation break down would produce a difference in brightness between the two lamps

Describe a 4 wire-3phase distribution system

The four-wire system uses a single return wire, which is connected to the neutral point of the star windings. Current in the return wire is the sum of currents in the individual phases. If the loads on each phase are balanced with voltages equal and 120° apart, the three currents will sum to zero and the return wire will carry no current. The return wire will carry a small current only if there is an imbalance.

Sketch an earth fault meter and explain how it works (WS El. p. 55)

The instrument works by injecting a stabilized voltage - typically 21.5V DC for systems up to 660V AC - via resistance R1 and a diode. "Re" represents the resistance caused by an insulation breakdown or earth fault on the system. When "Re" is infinite no current flows via resistor "R1" and so the voltage across "R1" is zero and hence the output is zero. As "Re" falls the current flowing via "R1" will increase and hence the output voltage will increase. The instrument gives both visual and audible indication of a fault, and can be set to trigger an alarm at a preset value of insulation resistance. A typical value for the "R1" resistor would be 750 ohms, with this value being selected to produce less than 30 mA output through a dead short.

Explain the term "single phasing." State its effects on a motor (WS p. 94)

The loss of current through one of the 3 phase AC supply lines is known as single phasing. Its effects are: 1) The current in the remaining two phases increases to 173% the normal current value. 2) The motor speed is reduced. 3) The motor becomes noisy and starts vibrating due to uneven torque produced in the motor. 4) If the motor is arranged for standby and automatic starting then the motor will not start, and if the overload relay provided fails to function then the motor may burn. 5) The windings will melt due to overheating and can give a fatal shock to the personnel.

Sketch an earth fault diagram explaining how it works. Describe how an earth fault is indicated by using an AC earth fault lamp system. (WS El. p. 54)

Two lamps are used, each rated for the full system voltage, but connected in series across the busbars with the mid point earthed. If the system is correctly insulated then both lamps will glow at half brilliance. A full earth on one pole will short circuit its lamp, causing the other to shine brightly. A slight insulation break down would produce a difference in brightness between the two lamps

Explain UPS

Uninterruptible Power Supply is an electrical apparatus that provides emergency power to a load when the input power source or mains power fails. A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide near-instantaneous protection from input power interruptions, by supplying energy stored in batteries, supercapacitors, or flywheels.

Explain why an AC motor may need a Star/Delta starter

With this method, the six connections to the three stator phase windings of the motor are brought out to the starter. For normal running the windings are connected in delta, but for starting they are star-connected. The star connection reduces the starting current, but also reduces torque. When the delta connection is established the motor will draw line current corresponding to its speed, which at 80% speed will usually be around 3.5 times full load current.

Outline the necessary precautions as stated in COSWOP for working near live electrical equipment when essential for the safety of the ship

a) A second person, who should be competent in the treatment of electric shock should be continually in attendance. b) The working position adopted should be safe and secure to avoid accidental contact with the live parts. Insulated gloves should be worn where practicable. c) Contact with the deck particularly if it is wet should be avoided. Footwear may give inadequate insulation if it is damp or has metal studs or rivets. The use of a dry insulating mat at all times is recommended. d) Contact with bare metal should be avoided. A hand-to-hand shock is especially dangerous. To minimise the risk of a second contact should the working hand accidentally touch a live part, one hand should be kept in a trouser pocket whenever practicable. e) Wrist watches, metal identity bracelets and rings should be removed. They provide low resistance contacts with the skin. Metal fittings on clothing or footwear are also dangerous.