Peter's Physics Pages

An introduction to Electricity and Strength of Materials with Peter Eyland

Lecture 5 (Electrical Safety)

In this lecture the following are introduced:
• Skin Resistance
• Hazardous currents
• The Australian Electrical Supply
• The Cleaning Lady and the Dog (A cautionary tale)
• Circuit Breakers and Fuses
• Switches
• Competent Persons
• Domestic Sockets and Plugs

Skin Resistance

The fluids in the tissues under the skin have a large number of ions (charged molecules) moving in them. This means that the inside of the human body will conduct electricity almost as well as metals. Measuring resistance between separate places on the skin usually give a large resistance because of poor contact between the probes and the skin. A square centimetre of dry skin can have a resistance up to 100kΩ, but wetting it reduces resistance considerably.

Aside:
In medicine, a conducting gel is used between the electrodes and the skin, in order to give good contact. Resistance is often measured by having an electrode on a leg and a "roller" to move the other electrode around the body. The current between them is inversely proportional to the resistance. A Carcinoma (or cancer) can be detected by a sudden increase in resistance as the roller approaches it.

Hazardous currents

It is the current that kills, not the potential difference, people have died from 12V sources. If the path of the current is from the hand to the same side foot, the threat to life is less than if the current crosses through vital organs in the body.

 1 mA leakage current 2 - 3 mA you feel a shock > 10 mA you have a painful shock 15 - 30 mA you can't move contracted muscles > 30 mA you have breathing difficulties 70 - 200 mA death by ventricular fibrillation > 200 mA your heart may restart

These are currents which are external onto the skin, internal currents are much more serious.

Because of the possibility that current may stop you releasing any contracted muscles, if you have no other recourse, you should test potential hazards with the back of your hand. If it is "live", the current will cause your hand muscles to contract and pull you away from the contact. If it is "live" and you touch it with your palm side, then your fingers will immediately contract into a (death?) grip that you cannot break.

In ventricular fibrillation, the heart beats rapidly and faintly without pumping blood around. If untreated by "defibrillators" (to stop the heart and then restart it) it will continue till death intervenes.

First Aid should start by switching off the current. If this is not possible then an insulating material (dry wood or rope) should be used to push or pull the person away from the current. You must avoid any electrical contact, or the current will pass through you.

Having isolated the person from the current, if the person is not breathing, artificial respiration should be started at once. If there is no heartbeat then cardiac massage (CPR) must be applied. Training courses in CPR and notices describing it are important in the workplace as even untrained people have saved lives by following the CPR notices instructions.

Current density (current/area) determines the size of damage to the skin and neighbouring tissues.

The Australian Electrical Supply

In Australia, electricity is supplied at 240V and 50Hz. Other countries have 110V or 220V or 60Hz.
Three wires are used.

 The Active wire (high potential) is coloured brown (used to be red). The Neutral wire (low potential) is coloured blue (used to be black). The Earth wire is striped green and yellow (used to be only green).

The colours were changed so that red-green colourblind people could live as electricians (i.e. not be dead).

The Neutral carries a current in normal operation.
The Earth is the local Earth potential (usually achieved by attaching it to a pipe in the ground at your house) and does not carry a current in normal operation.

The Neutral starts out as Earth potrential at the general supply to an area, but the resistance of each house creates a potential difference and so the potential in the Neutral increases with distance from its distant Earth.

Since the Neutral may differ in potential from Earth, what you think is safe may not be, and current from a frayed or faulty Neutral (or worse, the Active!) may leak to the frame of an electric device.

The (local) Earth is usually connected to the frame of metal devices, such as washing machines, to pass current to the pipe in the ground outside, and make sure that you (at local earth potential) can touch the outside of it without being electrocuted.

If current flows along the Earth wire, there is some fault in the system and the currents in the Active and Neutral will be different.

 Ground Fault Circuit Interupters (GFCI) have a switch which is intended to cut the power when the Active and Neutral currents are different and/or current flows in the Earth. GFCIs are also called "Residual Current Devices", "Earth Leak Trips" and "Safety Switches". The diagram shows that magnetic fields cancel when the active and neutral currents are equal. When they differ another magnetic field pulls the switches open.

The Cleaning Lady and the Dog (A coautionary tale)

Before GFCIs, in one hospital some surgeons were operating on a patient. They had an EKG monitor (for electrical heart pulses) connected to an electrical socket on one side of the theatre, and also an Intracardiac Blood Pressure monitor (with internal heart catheter) connected to an electrical socket on the other side of the theatre. Both were "Earthed" on the patient for safety.

Suddenly, a 50Hz potential appeared on the EKG and the patient's heart fibrillated. The surgeons quickly disconnected the monitors and the patient was revived. When they checked the monitors, the potential had disappeared.

They reconstructed the senario with a dog on the table instead of a human. The strange signal had disappeared. However, after a fortnight of testing, suddenly the mysterious signal reappeared. They rushed out of the theatre (leaving the dog) and found a cleaning lady with an electric scrubber in the next room. Examining the scrubber, they found that it was leaking 1A into the Earth leads.

This is how the mathematics worked out. Since they had connected the monitors to different electrical sockets there was 30m of Copper wire (radius 1mm) between the Earth connections of the monitors.

The Earth wire had a resistivity of 1.7x10-8Ωm, a length of 30m and a radious of 1mm.
It's resistance therefore equaled 1.7x10-8×30/(π0.0012) = 0.17Ω.

The 1A current through the wire produced 0.17V in the Earth wire between the two Earth connections. Between one Earth connection and the other Earth connection, the resistance through the heart was 3200Ω.

There was then a current of 0.17/3200 = 0.05mA through the heart. As this was an internal flow through the body it was enough to produce ventricular fibrillation.

Hospitals now have GFCIs that are much more sensitive than normal household devices, and other protective services.

Other protective devices in general, include relays, circuit breakers and fuses and electronic monitoring devices. They are designed to protect against over-current, over-voltage, and reversing polarity (in a 3 phase system).

Circuit Breakers and Fuses

Circuit breakers protect against too much current
There are (as at time of writing) three main commercial types.
• MCCB i.e. Molded Case Circuit Breakers for 15A to 800A.
• PCB i.e. Power Circuit Breakers for 600A to 4000A
PCBs less than 72.5kV are called LVPCB i.e. Low Voltage Power Circuit Breakers. These are "air breakers"
PCBs lmore than 72.5kV are called HVPCB i.e. High Voltage Power Circuit Breakers.
• ICCB i.e. Insulated Case Circuit Breakers
These have the construction characteristics of both MCCB and LVPCB and are for use in fixed mounted switchboards.

Circuit breakers can be
bimetallic strips which respond to heat by bending, thus breaking the circuit, or
electromagnetic which respond to a sudden rise in current, or
electronic with a Central Processing Unit (CPU).

Analogue devices sense peak current, and digital devices detect average (or r.m.s.) current.

Fuses are "self destruct" devices which protect by melting a thin wire of Lead-Tin-Antimony and breaking the current. Though a colleague reported a case where the current in one of his devices vapourised the fuse wire, creating a plasma, which was a good conductor and enhanced the current flow rather than stopping it!

Switches

Switches are designated by the number of circuits they control at once (the "poles") and how many closed positions they have (the "throws").

 Single Pole Single Throw Double Pole Single Throw Single Pole Double Throw Double Pole Double Throw

Competent Persons

Under the relevant acts, e.g. AS/NZS 3000:2007, only "competent persons" should construct or repair electrical equipment. A competent person is one who has acquired, through training, qualification or experience, or a combination of these, the knowledge and skill to enable that person to perform the required task correctly.

Domestic Sockets and Plugs

When connecting wires at the back of plugs and sockets, remember that the connections are mirror images of the front. Care should be taken to ensure that the active and neutral are not interchanged. Deaths have occurred because of this. Do not assume that they have been connected correctly.

 One illustration: In the top diagram on the right, a single pole single throw switch is connected in the active lead, so that any leakage should be at Neutral potential. If this switch was connected in the Neutral lead, any leakage could produce high potential in the external frame. A Double Pole Double Throw switch which controls both leads is now standard to try to prevent dangerous situations.

You may feel safe to work on the household electricals by pulling out all the fuses. However, as I have experienced, there can be a fault between the "fusebox" and the house connector to the street supply, which could leave all the house floating above earth potential. Always check for potential differences before handling anything electrical, even if you can't think how it could be live! You never know what someone has done before you.

Summarising:
 A square centimetre of dry skin can have a resistance up to 100kΩ, but wetting it reduces resistance considerably. 70 - 200 mA can produce ventricular fibrillation. In Australia, electricity is supplied at 240V and 50Hz. The Active wire (high potential) is coloured brown (used to be red). The Neutral wire (low potential) is coloured blue (used to be black). The Earth wire is striped green and yellow (used to be only green). Circuit breakers and Fuses protect against too much current.

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