Siemens SENTRON - Introduction

9
Protection through residual current protective devices
3.1 Additional protection (previously „Protection against direct contact“)
with I
Δn
≤30mA
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  owing
current – the internal resistance of the human body R
m
and the transfer resistance
of the location R
st
( gure1).
Direct contact
I
n
² 30 mA
R
B
R
st
R
m
I
M
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.