Tutorial: Mechanic - electrician

Topic: Basics of electrical engineering the 2nd. year

Protection in electric networksProtection of inanimate parts

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.

The inanimate part is a conductive part of the electric device, which can be touched and isn‘t usually electrified, but where in case of the malfunction a dangerous contact voltage occurs (for example on the metal casing of the distributor). The dangerous contact voltage is a voltage (or a variance of potentials), which can cause an injury by the electric current to the person or animal contacting the inanimate part during malfunction. Its size depends on the type of the environment, in which this contact happens (ČSN 33 2000-4-41).

The inanimate part is a conductive part of the electric device, which can be touched and isn‘t usually electrified, but where in case of the malfunction a dangerous contact voltage occurs (for example on the metal casing of the distributor). The dangerous contact voltage is a voltage (or a variance of potentials), which can cause an injury by the electric current to the person or animal contacting the inanimate part during malfunction. Its size depends on the type of the environment, in which this contact happens (ČSN 33 2000-4-41).

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

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

Extremely dangerous – wet, with the extreme corrosive aggression, places, where there is the 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.

areas permanently allowed contact voltage of inanimate parts AC DC

normal and dangerous 50 2) 120

extremely dangerous 1) 25 60

working in water without using tools, narrow spaces

12 25

1)Applies also for toys, some healthcare and other devices that may get in contact with human skin or hair or for premises, where level of danger demands it.2)Allowed ripple is 10%.

Prevents passing of the electric current through the body of the person or animal, or the current is reduced to the safe amount or the device in case of malfunction automatically disconnects from the source. This objective is assured by:An automatic disconnection from the source if a malfunction occurs.

Another level of isolation, so called additional isolation, which prevents the current passing, if there happens to be a breakdown of the basic layer of insulation (or rather the passing current is reduced to the safe amount).

Reducing of a malfunction current via the measure on the side of the source

Equalizing of the potential between inanimate parts and the environment, which causes reducing of the current amount passing through a body (applied in exceptional cases while meeting other protection measures).

Prevents passing of the electric current through a body of a person or an animal, or the current is reduced to the safe amount or a device in case of malfunction automatically disconnects from the source. This objective is assured by:An automatic disconnection from the source if a malfunction occurs.

Another level of isolation, so called additional isolation, which prevents the current passing, if there happens to be a breakdown of the basic layer of insulation (or rather the passing current is reduced to the safe amount).

Reducing of the malfunction current via the measure on the side of the source

Equalizing of the potential between inanimate parts and the environment, which causes reducing of the current amount passing through a body (applied in exceptional cases while meeting other protection measures).

For TN networks is valid: The grounding of inanimate parts is carried out by the connection of

an inanimate part using a protection conductor with the grounded point of the network, which is usually the node of the source. The principle of the automatic disconnection is described on the picture:

The path of the short circuit current is called the loop. The principle also implies the condition for an automatic disconnection in the TN network. For switching off, the impedance of the loop Zs has to be so small, that the short circuit current exceeds the amount of the current la ensuring an automatic action of the protection element in adequately short time. The short circuit current has the amount Uo/ Zs and has to be bigger than the current Ia. That implies the condition mentioned in ČSN 33 2000-4-41 Zs × Ia < Uo

What is the inanimate part

The dangerous contact voltage

Protection means for inanimate parts

The impedance loop

according to the picture: the path of the malfunction current and

the nature of the automatic disconnection in the TN network

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