General Safety Tips For Working With Electricity

What are some general safety tips for working with or near electricity?

Inspect portable cord-and-plug connected equipment, extension cords, power bars, and electrical fittings for damage or wear before each use. Repair or replace damaged equipment immediately.

Always tape extension cords to walls or floors when necessary. Nails and staples can damage extension cords causing fire and shock hazards.

Use extension cords or equipment that is rated for the level of amperage or wattage that you are using.

                                                                                                                   

Always use the correct size fuse. Replacing a fuse with one of a larger size can cause excessive currents in the wiring and possibly start a fire.

Be aware that unusually warm or hot outlets may be a sign that unsafe wiring conditions exists. Unplug any cords or extension cords to these outlets and do not use until a qualified electrician has checked the wiring.

Always use ladders made with non-conductive side rails (e.g., fiberglass) when working with or near electricity or power lines.

Place halogen lights away from combustible materials such as cloths or curtains. Halogen lamps can become very hot and may be a fire hazard.

Risk of electric shock is greater in areas that are wet or damp. Install Ground Fault Circuit Interrupters (GFCIs) as they will interrupt the electrical circuit before a current sufficient to cause death or serious injury occurs.

Use a portable in-line Ground Fault Circuit Interrupter (GFCI) if you are not certain that the receptacle you are plugging your extension cord into is GFCI protected.

Make sure that exposed receptacle boxes are made of non-conductive materials.

Know where the panel and circuit breakers are located in case of an emergency.

Label all circuit breakers and fuse boxes clearly. Each switch should be positively identified as to which outlet or appliance it is for.

Do not use outlets or cords that have exposed wiring.

Do not use portable cord-and-plug connected power tools with the guards removed.

Do not block access to panels and circuit breakers or fuse boxes.

Do not touch a person or electrical apparatus in the event of an electrical accident. Always disconnect the power source first.

What kinds of injuries result from electrical currents?

People are injured when they become part of the electrical circuit. Humans are more conductive than the earth (the ground we stand on) which means if there is no other easy path, electricity will try to flow through our bodies.

There are four main types of injuries:

Electrocution (Fatal), Electric shock, burns, and falls. These injuries can happen in various ways:

Direct contact with exposed energized conductors or circuit parts. When electrical current travels through our bodies, it can interfere with the normal electrical signals between the brain and our muscles (e.g., heart may stop beating properly, breathing may stop, or muscles may spasm).

When the electricity arcs (jumps, or "arcs") from an exposed energized conductor or circuit part (e.g., overhead power lines) through a gas (such as air) to a person who is grounded (that would provide an alternative route to the ground for the electrical current).

Thermal burns including burns from heat generated by an electric arc, and flame burns from materials that catch on fire from heating or ignition by electrical currents or an electric arc flash. Contact burns from being shocked can burn internal tissues while leaving only very small injuries on the outside of the skin.

Thermal burns from the heat radiated from an electric arc flash. Ultraviolet (UV) and infrared (IR) light emitted from the arc flash can also cause damage to the eyes.

An arc blast can include a potential pressure wave released from an arc flash. This wave can cause physical injuries, collapse your lungs, or create noise that can damage hearing.

Muscle contractions, or a startle reaction, can cause a person to fall from a ladder, scaffold or aerial bucket. The fall can cause serious injuries.

What are some tips for working with power tools?

Switch all tools OFF before connecting them to a power supply.

Disconnect and lockout the power supply before completing any maintenance work tasks or making adjustments.

Ensure tools are properly grounded or double-insulated. The grounded equipment must have an approved 3-wire cord with a 3-prong plug. This plug should be plugged in a properly grounded 3-pole outlet.

Test all tools for effective grounding with a continuity tester or a Ground Fault Circuit Interrupter (GFCI) before use.

Do not bypass the on/off switch and operate the tools by connecting and disconnecting the power cord.

Do not use electrical equipment in wet conditions or damp locations unless the equipment is connected to a GFCI.

Do not clean tools with flammable or toxic solvents.

Do not operate tools in an area containing explosive vapors or gases, unless they are intrinsically safe and only if you follow the manufacturer's guidelines.

What are some tips for working with power cords?

What is a Ground Fault Circuit Interrupter (GFCI)?

When and how do I test the Ground Fault Circuit Interrupter (GFCI)?

What is a sample checklist for basic electrical safety?

Inspect Cords and Plugs

Eliminate Octopus Connections

Never Break OFF the Third Prong on a Plug

Never Use Extension Cords as Permanent Wiring

Keep power cords clear of tools during use.

Suspend extension cords temporarily during use over aisles or work areas to eliminate stumbling or tripping hazards.

Replace open front plugs with dead front plugs. Dead front plugs are sealed and present less danger of shock or short circuit.

Do not use light duty extension cords in a non-residential situation.

Do not carry or lift up electrical equipment by the power cord.

Do not tie cords in tight knots. Knots can cause short circuits and shocks. Loop the cords or use a twist lock plug.

A Class A Ground Fault Circuit Interrupter (GFCI) works by detecting any loss of electrical current in a circuit (e.g., it will trip at a maximum of 6mA). When a loss is detected, the GFCI turns the electricity off before severe injuries or electrocution can occur.

A painful non-fatal shock may occur during the time that it takes for the GFCI to cut off the electricity so it is important to use the GFCI as an extra protective measure rather than a replacement for safe work practices.

GFCI wall outlets can be installed in place of standard outlets to protect against electrocution for just that outlet, or a series of outlets in the same branch circuit. A GFCI Circuit Breaker can be installed on some circuit breaker electrical panels to protect an entire branch circuit. Portable in-line plug-in GFCIs can be plugged into wall outlets where appliances will be used.

It is important that you follow the manufacturer's instructions with respect to the use of a GFCI.  Test permanently wired GFCIs monthly and portable devices before each use. Press the "test" and "reset" buttons.

Plug a "night light" or lamp into the GFCI-protected wall outlet (the light should turn on), then press the "TEST" button on the GFCI. If the GFCI is working properly, the light should go out. If not, have the GFCI repaired or replaced. Press the "RESET" button on the GFCI to restore power.

If the "RESET" button pops out but the "night light" or lamp does not go out, the GFCI has been improperly wired and does not offer shock protection at that wall outlet. Contact a qualified electrician to correct any wiring errors.

Check extension cords and plugs daily. Do not use, and discard if worn or damaged. Have any extension cord that feels more than comfortably warm checked by an electrician.

Do not plug several items into one outlet.

Pull the plug, not the cord.

Do not disconnect power supply by pulling or jerking the cord from the outlet. 

Pulling the cord causes wear and may cause a shock.

Replace broken 3-prong plugs and make sure the third prong is properly grounded.

Use extension cords only to temporarily supply power to an area that does not have a power outlet.

Keep extension cords away from heat, water and oil. They can damage the insulation and cause a shock.

Do not allow vehicles to pass over unprotected extension cords. 

Extension cords should be put in protective wire way, conduit, pipe or protected by placing planks alongside them.

Lightning Arrester And Their Types

A lightning arrester / lightning arrestor / lightning diverter is a device used on electric power systems and telecommunication systems to protect the insulation and conductors of the system from the damaging effects of lightning.

Working Principle of LA:

The earthling screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves, which may reach the terminal apparatus. The lightning arresters or surge diverts provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground.

The earthling screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves, which may reach the terminal apparatus. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground.

It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-linear resistance is that its resistance increases as the voltage (or current) increases and vice-versa.

The action of the lightning arrester or surge diverter is as under:

(i) Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting

(ii) On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.

(iii) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to overvoltage, the arc would be a short-circuit on the power system and may cause power-follow current in the arrester. Since the characteristic of the resistor is to offer low resistance to high voltage (or current), it gives the effect of short-circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.

Type of LA for Outdoor Applications:

There are several types of lightning arresters in general use. They differ only in constructional details but operate on the same principle, providing low resistance path for the surges to the round.

1. Rod Gap Arrester

2. Horn Gap Arrester

3. Multi Gap Arrester

4. Expulsion Type Lightning Arrester

5. Valve Type Lightning Arrester

(1) Rod Gap Arrester:

It is a very simple type of diverter and consists of two 1.5 cm rods.

One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators.

The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth.

Limitations:

(i) After the surge is over, the arc in the gap is maintained by the normal supply voltage, leading to short-circuit on the system.

(ii) The rods may melt or get damaged due to excessive heat produced by the arc.

(iii) The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester.

(iv)  The polarity of the f the surge also affects the performance of this arrester.

(v) Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main arresters.

(2) Horn Gap Arrester:

It consists of a horn shaped metal rods A and B separated by a small air gap. The horns are so constructed that distance between them gradually increases towards the top as shown.

The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and choke coil L while the other end is effectively grounded.

The resistance R helps in limiting the follow current to a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be protected.

The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the gap.

Under normal conditions, the gap is non-conducting i.e. normal supply voltage is insufficient to initiate the arc between the gap. On the occurrence of an over voltage, spark-over takes place across the small gap G. The heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves progressively into positions 1, 2 and 

3.

At some position of the arc (position 3), the distance may be too great for the voltage to maintain the arc; consequently, the arc is extinguished. The excess charge on the line is thus conducted through the arrester to the ground.

(3) Multi Gap Arrester:

It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one another and separated by small intervals of air gaps. The first cylinder (i.e. A) in the series is connected to the line and the others to the ground through a series resistance. The series resistance limits the power arc. By the inclusion of series resistance, the degree of protection against traveling waves is reduced.

In order to overcome this difficulty, some of the gaps (B to C in Fig) are shunted by resistance. Under normal conditions, the point B is at earth potential and the normal supply voltage is unable to break down the series gaps. On the occurrence an over voltage, the breakdown of series gaps A to B occurs.

The heavy current after breakdown will choose the straight – through path to earth via the shunted gaps B and C, instead of the alternative path through the shunt resistance.

Hence the surge is over, the arcs B to C go out and any power current following the surge is limited by the two resistances (shunt resistance and series resistance) which are now in series. The current is too small to maintain the arcs in the gaps A to B and normal conditions are restored. Such arresters can be employed where system voltage does not exceed 33kV.

(4) Expulsion Type Arrester:

This type of arrester is also called ‘protector tube’ and is commonly used on system operating at voltages up to 33kV. Fig shows the essential parts of an expulsion type lightning arrester.

It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each line conductor.

On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube walls resulting in the production of neutral gas. In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode, which is hollow. As the gas leaves the tube violently it carries away ionized air around the arc. This de ionizing effect is generally so strong that the arc goes out at a current zero and will not be re-established.

Advantages:

(i) They are not very expensive.

(ii) They are improved form of rod gap arresters as they block the flow of power frequency follow currents

(iii) They can be easily installed.

Limitations:

(i) An expulsion type arrester can perform only limited number of operations as during each operation some of the fiber material is used up.

(ii) This type of arrester cannot be mounted on enclosed equipment due to discharge of gases during operation.

(iii) Due to the poor volt/am characteristic of the arrester, it is not suitable for protection of expensive equipment.

(5) Valve Type Arrester:

Valve type arresters incorporate non linear resistors and are extensively used on systems, operating at high voltages. Fig shows the various parts of a valve type arrester. It consists of two assemblies (i) series spark gaps and (ii) non-linear resistor discs in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container.

The spark gap is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gap is line raised by means of additional resistance elements called grading resistors across the gap. The spacing of the series gaps is such that it will withstand the normal circuit voltage. However an over voltage will cause the gap to break down causing the surge current to ground via the non-linear resistors.

The non-linear resistor discs are made of inorganic compound such as thyrite or metrosil. These discs are connected in series. The non-linear resistors have the property of offering a high resistance to current flow when normal system voltage is applied, but a low resistance to the flow of high surge currents. In other words, the resistance of these non-linear elements decreases with the increase in current through them and vice-versa.

Working:

Under normal conditions, the normal system voltage is insufficient to cause the breakdown of air gap assembly. On the occurrence of an overvoltage, the breakdown of the series spark gap takes place and the surge current is conducted to earth via the non-linear resistors. Since the magnitude of surge current is very large, the non-linear elements will offer a very low resistance to the passage of surge. The result is that the surge will rapidly go to earth instead of being sent back over the line. When the surge is over, the non-linear resistors assume high resistance to stop the flow of current.

(6) Silicon Carbide Arresters:

A great number of silicon carbide arresters are still in service. The silicon carbide arrester has some unusual electrical characteristics. It has a very high resistance to low voltage, but a very low resistance to high-voltage.

When lightning strikes or a transient voltage occurs on the system, there is a sudden rise in voltage and current. The silicon carbide resistance breaks down allowing the current to be conducted to ground. After the surge has passed, the resistance of the silicon carbide blocks increases allowing normal operation.

The silicon carbide arrester uses nonlinear resistors made of bonded silicon carbide placed in series with gaps. The function of the gaps is to isolate the resistors from the normal steady-state system voltage. One major drawback is the gaps require elaborate design to ensure consistent spark-over level and positive clearing (resealing) after a surge passes. It should be recognized that over a period of operations that melted particles of copper might form which could lead to a reduction of the breakdown voltage due to the pinhole effect. 

Over a period of time, the arrester gap will break down at small over voltages or even at normal operating voltages. Extreme care should be taken on arresters that have failed but the over pressure relief valve did not operate. This pressure may cause the arrester too.

(7) Metal Oxide Arrester:

The MOV arrester is the arrester usually installed today

The metal oxide arresters are without gaps, unlike the SIC arrester. This “gap-less” design eliminates the high heat associated with the arcing discharges.

The MOV arrester has two-voltage rating: duty cycle and maximum continuous operating voltage, unlike the silicon carbide that just has the duty cycle rating. A metal-oxide surge arrester utilizing zinc-oxide blocks provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection. Failure is reduced, as there is no air gap contamination possibility; but there is always a small value of leakage current present at operating frequency.

It is important for the test personnel to be aware that when a metal oxide arrester is disconnected from an energized line a small amount of static charge can be retained by the arrester. As a safety precaution, the tester should install a temporary ground to discharge any stored energy.

Duty cycle rating: The silicon carbide and MOV arrester have a duty cycle rating in KV, which is determined by duty cycle testing. Duty cycle testing of an arrester is performed by subjecting an arrester to an AC rms voltage equal to its rating for 24 minutes. During which the arrester must be able to withstand lightning surges at 1-minute intervals.

Maximum continuous operating voltage rating: The MCOV rating is usually 80 to 90% of the duty cycle rating.

Installation of LA:

The arrester should be connected to ground to a low resistance for effective discharge of the surge current.

The arrester should be mounted close to the equipment to be protected & connected with shortest possible lead on both the line & ground side to reduce the inductive effects of the leads while discharging large surge current.

Maintenance of LA:

Cleaning the outside of the arrester housing.

The line should be de-energized before handling the arrester.

The earth connection should be checked periodically.

To record the readings of the surge counter.

The line lead is securely fastened to the line conductor and arrester

The ground lead is securely fastened to the arrester terminal and ground.