Page 58 - Electrical Safety of Low Voltage Systems

Mathematical Principles of Electrical Safety 41

Therefore, S IIBE (t) < S II (t) and the presence of a metal enclosure causes
k IIBE > k II and r IIBE (t) > r II (t).

3.9 Safety and Risk of Electrical Separation
The protective measure by electrical separation has been examined in
Chap. 2. In this section, let us assume an ordinary separation trans-
former (i.e., no double insulation between the primary and secondary
windings). Persons are at risk of electric shock if the basic insulations
of both the separation transformer and the ECP fail (Fig. 3.10).
Safety S ES (t) is given by Eq. (3.16):

TR ECP
S ES (t) = 1 − 1 − S (t) 1 − S (t)
BI BI
TR
TR
BI + e
= e − t − ECP t − e −( + ECP )t (3.16)
BI
BI
BI
where TR and ECP , respectively, indicate the failure rate of the basic
BI BI
insulations of the separation transformer and of the appliance.
S ES (t) is of the same magnitude as S II (t) given in Eq. (3.13). How-
ever, the risk of touch voltages is greater for the electrical separation
because of the presence of the metal enclosure of the ECP, which in-
creases the probability of contact, and therefore r ES (t) > r II (t).
Also for the electrical separation, the bonding of the enclosures
is not permitted, because the probability that the transformer and
the ECP fail is less than the probability that the bonding connection
dangerously energizes the ECP.

FIGURE 3.10 Risk as related to electrical separation.