Transcript AVOP-ELEKTRO-SKA-007

Tutorial: Mechanic - electrician
Topic:
Basics of electrical engineering
the 2nd. year
Protection in electric networks
Protection 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.
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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).
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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
source
of a malfunction current via the measure on the side of the
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
the source
of the malfunction current via the measure on the side of
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).
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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
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Zs × Ia < Uo
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What is the inanimate part
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The dangerous contact voltage
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Protection means for inanimate parts
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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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www.wikipedia.cz
www.vosaspsekrizik.cz/cs/download/padesat
ka/ochrana-dotyk.pdf
Elektro v praxi, 1 část, Solid Team