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Shocking Batteries

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The vast majority of batteries are used in low voltage applications where there is little danger of electric shock, but familiarity can breed a careless attitude towards the potential dangers.

With high voltage batteries the danger of electric shock is very real.

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As the use of batteries with voltages in excess of 300 Volts becomes more commonplace with the growing popularity of electric and hybrid electric vehicles, there is a danger that the general public, so used to relatively benign 12 Volt batteries, may underestimate the hazards associated with higher voltage traction batteries. Electric shocks account for about 1% of all fatal accidents, mostly to people who ought to know better.

This page describes the dangers and outlines some safety precautions when working with high voltage batteries.

 

Electric Shock

A physiologist may view the body as containing an electrical network, passing tiny nerve signals around enabling us to do all those essential things we like to do so much such as breathing, thinking and moving. Its function can be severely disrupted by the presence of an extraneous current. The body also contains a network of canals transporting oxygen to the muscles and the brain in a salty solvent called blood which incidentally provides a good conducting medium for electricity.

To the battery however, the body is simply an insulated skin bag containing electrolyte. See also nerve impulses.

 

Despite its common use as an indicator of danger, and the implication in the opening paragraph, voltage is not a reliable indicator of the severity of an electric shock. The most important indicators are the actual current which flows through the body and its duration, and even these can lead to misleading conclusions because the physiological consequences depend on the route the current takes through the body. Current passing through the heart or the brain is infinitely more damaging than current passing across a finger or the palm of the hand caught between the terminals of a battery. A sustained current will also do more damage than a short current pulse.

 

Physiological Consequences of Electric Shock

The table below outlines some of the effects of direct electrical currents passing through the body for a period of one second.

Important Notes: The two tables on this page are compiled from a variety of sources and although there is general agreement between the sources on the magnitude of the causes and effects, the actual values are subject to very wide variations. Obviously, it is not practical to perform tests on human subjects to verify the levels at which shocks become fatal and some data is derived test on animals. The values used are therefore average or typical values which should be used for illustrative purposes only.

Dangerous currents are shown in Red.


Corner

Shocking Effects

Corner

Current (contact 1 second)

Physiological Effect

Less than 1mA

No sensation

1mA

Threshold of feeling. Tingling sensation

5mA

Maximum harmless current

8 -15 mA

Mild shock

Start of muscular contraction.

No loss of muscular control

15 -20 mA

Painful shock

Sustained muscular contraction.

Can't let go of conductor

20-50 mA

Can't breathe. Paralysis of the chest muscles

Possibly Fatal

50 - 100 mA

Intense pain

Impaired breathing

Ventricular fibrillation

Possibly fatal - Fatal if continued

100-200 mA

Ventricular fibrillation

Probably fatal - Fatal if continued

Respiratory function continues

Over 200 mA

Sustained ventricular contractions followed by normal heart rhythm (defibrillation)

Chest muscles clamp the heart and stop it for the duration of the shock. This also prevents ventricular fibrillation improving the chances of survival, but other factors come into play.

Burns

Temporary respiratory paralysis.

Possibly fatal - Fatal if continued

Over 1 Amp

Severe burns.

Internal organs burned.

Death

Survivable if vital organs not in current path - e.g. across a finger or hand

 

Notes:

  • Low voltages do not mean low hazard.

  • Other things being equal the degree of injury is proportional to the length of time the body is in the circuit.

  • According to the IEEE Std. 80, the maximum safe duration of a shock can be determined by the formula

    T = 0.116/(E/R), where T is the time in seconds, E is the voltage and R, the resistance of the person (assumed to be 1000 ohms).

    For a 120V circuit the maximum shock duration = 0.116/(120V/1000) = 1 Second
    For a 240V circuit the maximum shock duration = 0.116/(240V/1000) = 0.5 Second

  • It is extremely important to free a shock victim from contact with the current as quickly as possible. The difference of a few seconds in starting artificial respiration may spell life or death to the victim. Don't give up unless the victim has been pronounced dead by a doctor.
  • Women tend to be more susceptible to electric currents than men
  • Lower body weight increases the susceptibility to electric currents
  • A shock from DC is more likely to freeze or stop the victim's heart.
  • The current range of 100 to 200 ma, is particularly dangerous because it is almost certain to result in lethal ventricular fibrillation, the shocking  of  the  heart  into  a useless flutter rather than a regular beat .
  • The fibrillation threshold is a function of current over time. For example, fibrillation will occur with 500mA over 0.2 seconds or 75mA over 0.5 seconds.

  • AC is more dangerous than DC causing more severe muscular contractions. AC is also more likely to cause a victim's heart to fibrillate , which is a more dangerous condition. Safe working thresholds are consequently much lower for AC voltages.
  • It is easier to restart a stopped heart once the source of the electric shock has been removed than it is to restore a normal beating rhythm to a fibrillating heart. A heart that is in fibrillation cannot be restored to normal by closed chest cardiac massage. Defibrillators give the heart a jolt of DC to stop fibrillation to allow the heart to restart with a normal beat.
  • Victims of a high voltage shock usually respond better to artificial respiration than do victims of a low voltage shock, probably because the higher voltage and current clamps the heart and hence prevents fibrillation. The chances of survival are good if the victim is given immediate attention.
  • Shock victims may suffer heart trouble up to several hours after being shocked. The danger of electric shock does not end after the immediate medical attention.
  • Don't expect an earth leakage trip or ground fault detector (circuit breaker) to protect you. They usually trip at 15 Amps.

 

Shocking Potential

While the severity of the electric shock is mainly determined by the current, the current in turn is influenced by numerous variables which make up the resistance of the current path making it difficult to predict the current which will flow from a given voltage. The two major components of the resistance are, the resistance of the body between the points of contact with the electrical circuit, and the contact resistance between the body and the voltage source. In more detail the body resistance depends on the length of the conducting path through the body and the body weight. The contact resistance depends on whether the contact is wet or dry, the area of the contact, the firmness of the grip or touch of the electrical contact and whether there is any other insulation in the path. Because of this wide variation in resistance and contact duration, people have been known to survive shocks of 40 KV while others have been killed by less than 50 Volts

 

The following table shows the conditions which could lead to a serious electric shock. It gives the body and contact resistances associated with a variety of conditions and indicates the current which will flow for different voltages. The table above outlines the consequences.

 

Corner

Currents Resulting From Electric Shocks

Corner

Circumstance

 

Resistance

(Ohms)

Current mA

  50V  

100V

250V

500V

Hand to ground (Rubber gloves or soles)

20,000,000

0.002

0.005

0.01

0.02

Hand to ground (Dry hand, Leather soles)

1,000,000

0.05

0.1

0.2

0.5

Dry skin

500,000

0.1

0.2

0.5

1

Light touch (Dry)

500,000

0.1

0.2

0.5

1

Hand gripping wire or metal tool (Dry)

20,000

2.5

5

12.5

25

Light touch (Wet)

10,000

5

10

25

50

Hand to ground (Wet hand, Damp leather sole)

10,000

5

10

25

50

Hand gripping wire or metal tool (Wet)

5,000

10

20

50

100

Hand to Hand (Damp)

1500

33.3

66.6

166

333

Wet skin

1000

50

100

250

500

Across human body

1000

50

100

250

500

Hand to foot, Hand to hand (Excluding skin)

500

100

200

500

1000

Between the ears , Across a finger (Excluding skin)

100

500

1000

2500

5000

Punctured skin with cuts, abrasions or burns caused by the electric current itself

No resistance

Very high

Very high

Very high

Very high

 

Notes:

  • The skin is a most important insulator.
  • There are huge variations in contact resistance.
  • Working with minor wounds to the hands seriously increases the risk of shock.
  • Once a shock has been initiated, the resulting electrical burn can puncture the skin and increase the shocking current.
  • Rings, bracelets and other jewellery decrease the contact resistance to the body and increase the potential for electric shock.
  • Use only one hand (keeping one hand in your pocket) while working on high voltage circuits avoids the risk of the body becoming part of the circuit.
  • Risks can be minimised by using insulated hand tools (pliers, screwdrivers, spanners etc.) and by wearing rubber gloves and shoes.

 

Automotive Battery Circuit Design

Standard 12 Volt and 24 Volt automotive battery circuits rely on the vehicle chassis as the ground (earth) return circuit. This practice is acceptable when no lethal voltages are involved but not for high voltage batteries used in EV and HEV applications since someone working on the battery could easily become the conduit between any of the exposed the high voltage terminals (on the battery, the motor and its controller) and the chassis.

EV and HEV battery circuits should therefore use isolated battery busses for both the positive and negative sides of the battery.

This is also an essential operating safety feature since accidental loss of isolation could subject the driver or emergency services to hazardous voltages or cause a dangerous short circuit of the battery. In case of isolation failure or accidental damage, ground fault monitoring which detects current leakage from the battery or inertia switches which detect high G deceleration due to an accident, should automatically disconnect the battery.

 

Safe Working Practices

There are many published standards for safe working practices on AC power circuits including those from the HSE in the UK and OSHA in the USA. See the section on Standards.

The following are additional recommendations relevant to working on battery circuits.

  • The usual safe working voltage threshold for working on batteries is 50 volts DC.
  • Disconnect the battery from its load with an obvious, visible disconnection, not simply a switch whose position can be overlooked.
  • Avoid working on a fully charged battery. Arrange for batteries to be discharged wherever possible.
  • Use a voltmeter to check whether the battery is charged before starting work.
  • Use a continuity tester to check that there is no connection between either of the battery terminals and the equipment or vehicle chassis.(See above)
  • Check that any capacitors associated with the battery circuit are discharged.
  • Cover exposed terminals where possible to avoid touching or dropping tools onto them.
  • When starting work on a supposedly dead circuit make contact first with the back of one hand, so that if a shock should occur, the muscle reaction will not cause the hand to grip the conductor.
  • See also the notes above.

 

Treat high power batteries with the same respect you would give to AC mains voltages.

 

See also Short Circuits and their Consequences

 

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