Electrical Safety
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74 Terms
What are the three main types of electrical hazard covered in the lecture?
Static electricity
Dynamic/current electricity
Electricity and the human body
Can static electricity be dangerous?
Yes, conditionally. Static shocks can be very high in voltage but usually last such a short time that they are not normally dangerous. However, a static spark can cause a fire or explosion in a volatile environment.
Why can a static electricity shock have a very high voltage but still usually be less dangerous than mains electricity?
The current flows for a very short time, limiting its effect on the body
What is the main danger of static electricity in a volatile environment?
A spark can ignite flammable substances, causing a fire or explosion
What two factors are particularly important when considering the danger of an electric shock?
The current flowing through the body and the duration/path of the current. The lecture also notes that effects depend on frequency and individual circumstances.
What is the approximate resistance of dry human skin according to the lecture?
It can be over 100,000 Ω
What can happen to the resistance of the human body when the skin is wet?
It can fall to less than 1,000 Ω
Why does wet skin increase the danger of electric shock?
Wet skin has a much lower electrical resistance, allowing a greater current to flow through the body for the same voltage.
What is the basic relationship between voltage, current and resistance?
Ohm’s Law:
I = V/R
where:
I = current in amperes (A)
V = voltage in volts (V)
R = resistance in ohms (Ω).
What can approximately 1 mA of AC current through the body cause?
A small tingling sensation. It may cause a person to fall or drop something.
What can approximately 5 mA of AC current cause?
A slight, disturbing shock, although the average person can still let go.
What can approximately 10–20 mA of AC current cause?
A painful shock with loss of muscular control, potentially preventing the person from releasing the conductor.
What can approximately 18 mA or more cause?
Extreme pain and severe muscle contractions. Chest muscles may contract and breathing may stop (respiratory arrest).
What can approximately 100 mA or more cause?
Ventricular fibrillation, which rarely stops spontaneously.
What can a 10 A current cause?
Cardiac arrest, severe burns and probable death.
What is electrocution?
Death or severe injury caused by electric shock.
A person is exposed to 230 V and has a body resistance of 1,000 Ω. What current flows?
I = V/R
I = 230/1000
I = 0.23 A = 230 mA
Why is a 230 mA current through the body extremely dangerous?
According to the lecture’s current-effects table, it is in the range associated with ventricular fibrillation, as well as potentially severe shock and respiratory arrest.
What are the four protective methods covered in the lecture?
Earthing exposed metal parts
Insulation
Double insulation
Barriers, enclosures and distance.
What is the purpose of earthing exposed metal parts?
To provide a safe, low-resistance path for fault current to flow to earth, helping protective devices disconnect the circuit rather than allowing current to flow through a person.
What are the three conductors shown in a typical earthed circuit?
Live
Neutral
Earth
What is the normal role of the neutral conductor?
It provides the normal return path for current.
Why should you not assume that neutral is completely safe?
The lecture states that neutral is normally close to earth potential but should not be assumed to be safe.
Why is touching a live conductor dangerous?
A live conductor can be at around 230 V relative to earth, allowing current to flow through a person to earth.
What happens if a live wire contacts the exposed metal casing of an earthed appliance?
A fault current flows through the earth connection, producing a large current that causes the fuse/MCB to disconnect the circuit.
What type of equipment has exposed metal parts that are earthed and basic insulation?
Class 1 equipment.
What is the purpose of insulation?
To cover current-carrying and exposed metal parts and prevent contact with electrically live components
What is double insulation?
A protective method using two layers of insulating material around live components so that one failure does not expose a dangerous potential
Does double-insulated equipment require an external earthed metal casing?
No. It provides protection without an external earthed metal casing
What class of equipment uses double insulation?
Class 2 equipment.
What is the symbol for double-insulated/Class 2 equipment?
A square within a square.
When might barriers, enclosures or distance be used as protection?
When insulation is impractical or inadequate, distance and inaccessibility can be used to prevent contact.
What is the purpose of a fuse?
A fuse is a circuit protective device that protects cables and appliances from excessive current.
How does a fuse work?
It contains a wire with a low melting point. If the current exceeds the fuse rating, the wire melts and breaks the circuit.
Why is the fuse rating important?
It is selected so the cable/appliance should not be exposed to a current high enough to cause a fire.
Label fuse
What should you do after a fuse blows?
Replace it with a fuse of the same rating.
Should you repair or bypass a fuse?
No. The lecture explicitly says never repair or bypass a fuse.
What does MCB stand for?
Miniature Circuit Breaker.
What is the purpose of an MCB?
It is a circuit protective device that disconnects a circuit when an overcurrent occurs
What happens when an MCB trips?
It opens the switch and disconnects the circuit.
What is one major difference between a fuse and an MCB?
A fuse is destroyed when it operates and must be replaced, whereas an MCB can be reset.
What does RCD stand for?
Residual Current Device.
What does an RCD detect?
An imbalance between the current leaving through the live conductor and the current returning through the neutral conductor
Why can an imbalance between live and neutral indicate an electric shock?
Some current may be leaking from the normal circuit path, potentially through a person to earth.
What happens when an RCD detects a sufficient current imbalance?
It disconnects the circuit
What is the common rated residual operating current of an RCD used for personal protection?
30 mA
At 30 mA, how quickly must the RCD disconnect according to the lecture?
Within 300 ms. At higher residual currents, it operates faster
What is the purpose of the TEST button on an RCD?
It deliberately creates a current imbalance to check that the RCD trips correctly.
How often does the lecture recommend testing an RCD?
At least every 3 months
What is the difference between an RCD and a fuse/MCB?
Fuse/MCB: protect against excessive current/overcurrent.
RCD: detects current leaking from the normal live–neutral path by detecting an imbalance
Why are water and electricity particularly dangerous together?
Water can reduce body resistance, allowing a much larger current to flow through the body. The lecture specifically highlights kitchens, bathrooms, toilets and rain as hazards
What electrical hazards should you look out for with appliances?
Broken wiring
Damaged plugs/sockets
Old or damaged appliances
Overloaded sockets
What should you do with a faulty fuse?
Use the correct type, replace like with like, and never repair or bypass it
What should you NEVER do if someone is still in contact with an electrical source?
Do not touch them, because you may also become part of the electrical circuit
What should you do first if someone is being electrocuted?
Disconnect/isolate the electrical supply if it is safe to do so — e.g. using a switch, MCB or emergency stop.
When can you safely touch the victim?
Once you are certain they are disconnected from the electrical source and the scene is safe.
What should you do after the electrical source has been isolated?
Check responsiveness, call emergency assistance if needed, and assess breathing. If the person is unresponsive and not breathing normally, start CPR and use an AED when available.
What does the lecture say about medical assistance after a serious electrical emergency?
Call for medical assistance immediately.
What should you do if an electrical-injury victim is unresponsive and not breathing normally?
Start CPR and use an AED as soon as one is available, following appropriate first-aid/BLS training and local procedures.
What is microshock?
A potentially dangerous electrical current reaching the heart through a direct conductive pathway, such as a catheter or electrode
Why are patients with conductive catheters or electrodes particularly vulnerable to microshock?
They can provide a direct pathway to or near the heart, bypassing some of the body’s normal resistance.
How much current can potentially cause ventricular fibrillation when applied directly to the heart?
The lecture states that only tens of microamps (µA) can present a risk
How does microshock differ from an ordinary external electric shock?
With an external shock, the current must pass through the body’s tissues and skin. With microshock, a direct conductive pathway to/near the heart can allow very small currents to affect the heart.
What cardiac problem can microshock cause?
Ventricular fibrillation
What is done in higher-risk medical locations to reduce dangerous voltage differences?
Accessible conductive parts around the patient may be bonded together (equipotential bonding)
What electrical protection may be used in higher-risk medical locations?
isolated medical electrical supplies
continuous insulation monitoring
protective earthing
equipotential bonding.
What protects a circuit from excessive current?
Fuses and MCBs.
What detects leakage/current imbalance?
RCD
What protects exposed metal equipment by providing a path for fault current?
Protective earthing.
What protects against contact with live components by covering them?
Insulation
What type of equipment uses double insulation?
Class 2
What type of equipment has exposed metal parts connected to earth?
Class 1.
What is the most important difference between milliamps and microamps in electrical safety?
Normal external shocks can become dangerous at tens of milliamps, whereas with a direct pathway to the heart, tens of microamps can present a risk of ventricular fibrillation.