Additional Protection (Previously „Protection Against Direct Contact") With I; Δn - Siemens SENTRON Instructions Manual

Protection through residual current protective devices
3.1 Additional protection (previously „Protection against direct contact")
with I ≤ 30 mA
Δn
Additional protection is understood to mean protection which takes effect if there
is direct contact of a human body with a part normally live during operation in
the event of basic and/or fault protection failing. When a human body touches
live parts, two resistors connected in series determine the intensity of the fl owing
current – the internal resistance of the human body R
of the location R (fi gure1).
st
Direct contact
R
B
Figure 1: Protection against direct contact
In order to consider the accident properly, the worst case scenario where the
transfer resistance of the location is near zero must be assumed.
and the transfer resistance
m
I
² 30 mA
n
R
m
I
M
R
st
9