Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 110)

For switchgear itself, in some countries there are regulations with respect to the accessibility of
live components. This has resulted in a de facto standard for the devices that largely fulfill the
requirements of protection type IPXXB (finger safe). This considerably reduces the risk of an
electric shock by direct contact even when work is being carried out in switchgear assemblies.
For devices with larger rated currents, often protective covers are required for complying with
IPXXB.
4.1.1.2 Protection against indirect contact
Protection against indirect contact ensures that even in the event of failures – for example in
case of a conductive connection between a pole conductor and a metallic component – no
dangerous touch voltages (
≥ 50 V a.c. or ≥ 120 V d.c.) can arise or that such voltages in are
disconnected in a very short time before a risk of personal injury can result. Usually such
protection is achieved by grounding measures and short-circuit protection equipment such as
fuses, miniature circuit breakers or circuit breakers, as the fault currents in such cases can be
very large.
Protective Extra Low Voltage (PELV) and Safety Extra Low Voltage (SELV) are also suitable
means of protecting from indirect (and direct) contact. They are frequently used in electronic
circuits. See also Section
2.3.11 .
If the lines are long between the short-circuit protection device and the location of fault, the fault
current can fall below the response level of the short-circuit detecting device (for example of a
circuit breaker for motor protection) due to the damping effect of the line. For fix installed loads
(for example motors) the requirement is that the contact voltage on the motor-case – if
≥50 V
a.c. or ≥120 V d.c. – must be switched off within 5 seconds. To test compliance with these
conditions at the stage of engineering, the current in the event of fault has to be calculated
taking into account all loop impedances (incl. for example the internal resistance of the bimetal
tripping mechanism of any motor protective circuit breakers). The prospective tripping time
should be checked based on the overload characteristic of the protective device (
Fig. 4.1-1 ).
The manufacturer of protective devices offer advice on applications in such cases.
L
M
3
I> I> I>
Impedance of
the starter
Impedance of
the supply
network
Impedance of
the motor
supply lines
Fig. 4.1-1
When the cables to the load are long, impedances in the circuit can, in the event of a short-circuit in the
load, cause the fault current to fall below the tripping level of the short-circuit detector. It must then be
ensured that the thermal release mechanism clears the fault within 5 seconds.
LVSAM-WP001A-EN-P - April 2009 4-2