Tutorial: Mechanic - electrician

Topic: Basics of electrical engineering the 2nd. year

Protection in electric networks

Prepared by: Škarka Miloš

Projekt Anglicky v odborných předmětech, CZ.1.07/1.3.09/04.0002

je spolufinancován Evropským sociálním fondem a státním rozpočtem

České republiky.

Injury by electric current – physiological effect of electric current coming through a body of a person or an animal

Basic protection – protection against an injury by electric current in a failure-free state

Protection in the case of a malfunction – protection against an injury by electric current during one malfunction

Dangerous animate part – an animate part, which could cause an injury by electric current in certain conditions

Basic insulation – insulation of dangerous animate parts providing basic protection

Additional insulation – separate insulation used for basic insulation, to provide protection in case of a malfunction

Double insulation – insulation containing basic and additional insulation

Links – electric connections between conductive parts for potential balancing

Animate part – conductor of conductive parts designed for voltage in normal operation including the central conductor

Inanimate part is conductive part of an electric device, which can be touched and where there is usually no voltage, but where can a dangerous contact voltage occur because of a malfunction (for example metal casing of a distributor).

Dangerous contact voltage is voltage (or a variance of potentials), which can cause an injury by electric current to a person or an animal contacting an inanimate part during a malfunction. Its size depends on the type of environment, in which this contact occurs (ČSN 33 2000-4-41).

Allowed contact voltage is a part of

grounding system voltage opposite to the

ground, which can be crossed over by a

human in distance from hands to feet

(horizontally 1m).

Amount of the allowed contact voltage depends not only on a purpose of its use and on an environment, but even on a duration of a malfunction. The shorter the duration of the contact voltage is, the higher the contact voltage can be.

Allowable values of the permanently allowed contact voltage at the device up to 1000V are mentioned in the following chart.

Environment contact with parts (in operation)

Safe low voltage of animate parts [V]

Alternating Direct

Normal Animate 50 100

Inanimate 50 120

Dangerous *) Animate 25 60

Inanimate 50 120

Extremely Dangerous

Animate 12 25

Inanimate 25 60

*) It can be also specified differently in the regulation, if the sum of conditions (determining or influencing danger of an injury caused by an electric current) demands it

Normal – ordinary; cold; dusty (if dust is not conductive and not flammable). Its character prevents an injury by electric current.

Dangerous – hot, wet (even temporarily), dusty (if dust is conductive and not flammable with increased corrosive aggression), with shocks, outdoor places with mechanical damage and conductive environment. These are those places, where there is a temporary or permanent danger of an injury caused by electric current because of external effects.

Extremely dangerous – wet, with extreme corrosive aggression, places, where there is danger of an injury greatly increased by adverse conditions (water, kettles and metal tanks, narrow spaces with metal materials, healthcare facilities or special regulations for specific protection). The danger of an injury is even more increased because of special conditions and outer effects

Protection against a dangerous contact with animate parts

Prevents passing of electric current through a body of a person or an animal, or the current is reduced to the safe amount

Protection against a dangerous contact with inanimate parts

Prevents passing of electric current through a body of a person or an animal, or the current is reduced to the safe amount or the device in case of a malfunction automatically disconnects from the source.

Protection against a dangerous contact with animate and inanimate parts

Voltage in animate parts can never exceed the safe amount

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Inanimate part Dangerous contact voltage Allowed contact voltage of inanimate parts

types of environment Give examples

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ka/ochrana-dotyk.pdf Elektro v praxi, 1 část, Solid Team