Electricity - NEBOSH Quick Revision Sheet

Electrical Hazards / Electrical Terms
Primary / Secondary / Electrical current – Amps (A or I)
Voltage (potential difference) – Volts (V)
Resistance – ohms (R or Ω)
The electrical current that flows through the body is what will kill us with as little as 0.03 to 0.05 of an Amp or 30-50mAmps being capable of stopping the heart.

  • Involuntary muscular contraction (aka electric shock)
  • Ventricular fibrillation
  • Electricity passing through the body stopping the heart or causing respiratory arrest.
  • Skin tissue burns often from arcing
  • Deep tissue burns from current flowing internally through the body
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  • Arcs and sparks igniting flammable atmospheres causing explosions
  • Arcs and sparks setting light to flammable materials
  • Overheat electrical appliance and cables causing fires.
  • Muscular contraction causing person to fall or to drop an object
  • Shorting out circuits plunging the environment into darkness.

Situations or conditions that could affect the consequences of an electric shock / Actions to be taken if a person is suspected of having received an electric shock.
  • The age and physical characteristics of the individual, people with a pre-existing heart condition would be at greater risk.
  • The voltage the person comes into contact with, the higher the voltage the higher the potential current flow.
  • The protective devices fitted to the circuit and how effective they are at protecting people.
  • The persons resistance to neutral or earth return path. Wet environments would potentially reduce the resistance and thereby increase the potential current flow. Standing on a rubber mat would increase the resistance.
  • The personal protective equipment being worn such as rubber soled footwear and rubber insulating gloves.
  • The entry and exit path of the electric current, a path across the heart being the most dangerous.
  • The length of time the individual is in contact with the supply, the longer the time the greater the harm that will be suffered.
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  • Isolate supply immediately – Dial 999 or call for help (e.g. 1st Aider)
  • If you cannot isolate DO NOT attempt to touch casualty
  • Physically remove victim using non-conducting implements (e.g. wooden chair, wooden/plastic broom)
  • Check for pulse/breathing, give artificial respiration if necessary (CPR) and qualified to do so.
  • If they have a pulse and are breathing place in recovery position
  • Treat burns by covering with sterile dry lint free dressing, not uncommon these days to use cling film which gives good protection and will not adhere to the burn.
  • If victim regains consciousness, treat for physical shock; keep them rested and calm, do not give drinks, but maybe moisten their lips.
  • Remain with the injured person until medical aid arrives, 1st Aider or Paramedic.

Electricity – Control Measures
  • Isolation:
  • Before working on electrical systems, switch off the supply and display appropriate signage and lock it off with a padlock (each person working on the system would also lock if off with their own padlock) and padlock owner retains the key. Before commencing work, use a voltmeter to check the circuit is dead and also test the voltmeter on a known live circuit to prove it is working. Sometimes, an additional control of “earthing” the feed cable is use just in case the circuit somehow is re-energised (e.g. and induced voltage from an adjacent feed).
  • Insulation:
  • Ensuring all conductors or live parts are covered with insulation so if handled they do not cause electric shock e.g. cables on portable electrical equipment.
  • Note, overhead power lines generally are not insulated, but are placed at a distance where normally they cannot be touched.
  • Fuses:
  • A deliberate weak link in an electrical circuit so that if the appliance or circuit draws too much current the fuseable wire overheats and melts breaking the supply.
  • Fuses, however, are not an effective means of protecting people. If a circuit with a 5Amp fuse draws 5.1 Amps, the fuseable wire will overheat and melt, but will take a finite amount of time. The 0.1 Amps overcurrent (100 m.Amps) is enough to kill someone and the person could die before the fuse breaks the circuit supply.
  • Circuit Breakers:
  • Usually fitted to distribution boards, these are fast switching over-current trip devices which are specifically designed to protect people. If they trip they are easily reset without putting a person at risk, and if the fault is still present, will not be able be reset.
  • Residual Current Devices (RCD’s):
  • These devices work by monitoring the current being fed into a circuit on the live wire and comparing it against the current coming out of the circuit on the neutral wire. If they are the same, then the integrity of the circuit is sound so the supply is maintained. However, if there is a difference, then some current is going somewhere else e.g. through a person’s body to earth, so the device will trip extremely fast thus cutting off the supply. This usually happens at 30 m.Amps or less.
  • Advantages – does not need anyone to do anything for it to work, switches extremely fast at very low current differences usually before the person even senses an electric shock
  • Disadvantages – does not protect for overcurrent situations, so the circuit would still need a circuit breaker in it, and you will not know if it is serviceable or not (i.e. if it will work when needed) unless you implement a regime of testing RCD’s (usually on a before use for plug situated ones, or monthly for distribution board mounted items) to prove it’s functionality.
  • Earthing (appliances with earth connections are known as Class I equipment):
  • Here, exposed potentially conducting parts of equipment (e.g. chrome casings) are bonded to earth either by directly being wired into the buildings earth connection or via the earth pin on a 3-pin plug. If a fault occurs and the electricity shorts to the earthed parts, the circuit’s protective devices will activate and isolate the supply.
  • Three pin plugs are designed such the earth pin is the longest such that it is the first to be connected before the live and neutral, and if the plug is pulled out of the socket it is the last to be disconnected.
  • When wiring 3-pin plugs, the live wire is the shortest, the neutral is the second longest and the longest is the earth wire. This design feature ensures that if the cable is pulled out of the cable clamp and out of the plug, the first to be disconnect is the live, the second the neutral and last the earth which is the safety connection.
  • Double Insulation (appliances with double insulation are known as Class II equipment):
  • These items of equipment do not have an effective earth, usually just a plastic pin that allows them to be connect to a 3-pin socket. Some items only have 2-pins such as electric tooth brushes and shavers that utilise the 2-pin outlets in bathrooms. Here, safety is achieve by having two lots of insulation covering potentially live parts, and inner layer and an outer layer. If one of the layers is compromised, then the user is not put at risk of an electric shock.
  • Advantages – If you are in a situation where the earth continuity could be suspect e.g. running off a generator, or in a developing country this could result in a situation where the earth protection is compromised. Additional, not have the earthing connections within the appliance can reduce its physical size.
  • Reduced Voltage (110V CTE [centre tapped earth] or batter powered tools):
  • If a person (who normally has a skin resistance to electrical current of 2000 ohms touches a 230V supply cable then 115 m.Amps could flow through their body – enough to stop their heart or breathing. However if a person touches an 110V CTE cable (110V being made up of +55V and -55V, then, from 55V a current flow of 27.5 m.Amps can only be achieve, not enough to kill. Battery voltages are even lower so even safer.
  • Qualified Electricians to Carry Out Works:
  • All servicing and repairs must be undertaken by a suitably qualified electrician. For fixed wiring faults, a test certificate may need to be issued (e.g. minor works certificate).
  • Maintenance and Testing:
  • There is a requirement in the Electricity at Work Regulations for any protective devices or protective systems to be fit for purpose and subject to maintenance. Portable appliance testing, although not a legal requirement, is considered best practice when it comes to ensuring portable electrical equipment is in good condition and safe to use.

User’s Inspection Checks / Additional Formal Inspection Checks
  • Check mains cable for any damage to insulation such as cuts and nicks.
  • Check to ensure no bare wires visible (REPAIRED by a competent person)?
  • Ensure the cable not too long (potential trip hazard) or too short (could put undue stress on cable clamp potentially pulling out he wires
  • If subject to a PAR regime, checking PAT label attached to the equipment is in date.
  • Check that outer case of the equipment is undamaged nor loose and that all screws in are in place.
  • Check for any signs of overheating or burn marks on the equipment, sockets, cable or plug.
  • Check trip devices (RCD’s) working effectively by pressing the ‘test’ button.
  • Check the cable is securely clamp at both the plug and the entry point into the tool casing.
  • Check plug cover is secure and tight.
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  • Continuity checks of live, neutral and earth cables between plug and appliance (would indicate if there are any poor connections of cable damage).
  • Resistance breakdown tests between earth/live, earth neutral and neutral/live indicating potential cable breakdown faults.
  • Inspection of the fuse to ensure the correct rating is fitted (might need the plug to be dismantled).
  • Detailed examination of damage to both plug and appliance.
If any problems are found item should be quarantined for qualified electrician to repair.
Portable Appliance Testing Frequencies / Fixed Wiring Inspection and Testing Frequencies
  • Guidance given in HSG 107
  • Construction sites:
–110 V ac ~ before first use and thereafter 3-monthly
–230 V ac ~ before first use and thereafter 1-monthly
  • Other environments:
–Between 6-monthly (e.g. cooking equipment) up to 5-yearly (e.g. desktop computer systems.
–Double insulated equipment would not be tested, but subject to a formal visual inspection. /
  • Low risk (e.g. office environments) ~ 5 yearly
  • Medium risk (e.g. manufacturing environments) ~ 2 yearly
  • Special risk (e.g. petrol stations) ~ yearly
Design engineer would actually state their recommended intervals for this testing.
Requirements for Live Working
  • Live electrical working can only be carried out where:
–It is unreasonable to make live parts dead(such as during commissioning and testing for which the power needs to be switched on);
–It is reasonable to allow live working(voltage level does not give rise to undue risk and/or appropriate safe systems of work are in place); and
–Suitable safety precautions are in place(circuit appropriately protected by circuit safety devices; circuit breakers, RCD’s etc).
Safe Systems of Work
Buried Services / Overhead Services
-A permit to dig has been issued and authorised
-Correct drawings have been obtained
-CAT(Cable Avoidance Tool) scan of area by competent operators
  • Hum detection:
  • Energised cables will produce a magnetic field
  • Radio Frequency:
  • Telecoms cables will emit a radio frequency of the signal being carried
  • Transmitter and Receiver (signal and generator)
  • A signal is induced onto the line via a couple and then detected by a receiver
  • Mark the positions of buried services using marker spray, warning notices and/or stakes (non-conductive)
  • Hand dig trial holes to confirm the depth and position of buried services
  • Continue to scan as you dig
  • When digging use bucket with smooth edge not a toothed bucket.
  • Support and protect any exposed cables, do not attempt to move or reposition them, or used them to climb in and out of the excavation.
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  • Consult with local Electricity Company to obtain advice on the cables identifying the poles/towers you will be working close to.
  • Wherever possible ensure the lines are dead and earth to prevent them being re-engerised.
  • If they cannot be made dead establish barriers and warning signage at a safe distance from the power lines as directed by the distribution company. Ensure to work equipment can slew into this area; work facing the hazard.
  • If passing underneath power lines, set up goal posts to indicate safe clearance distances for vehicles passing underneath.
  • Ensure vehicles passing underneath cannot activate and jibs or buckets etc. that could narrow the distance between the vehicle and the line. If necessary this may need the ground level to be excavated out to make it lower.
  • Never store materials under power lines and it would indicate that you are going to bring in equipment to pick it up.
  • If safe clearance distances cannot be achieved then the use of cable covers might be needed to give some temporary insulation.