Page 54 - Electrical Safety of Low Voltage Systems

Mathematical Principles of Electrical Safety 37

FIGURE 3.6 Electrical
item with no
enclosure and basic
insulation.

The safety of an item, at any time, as offered by the basic insulation,
is estimated as shown in Eq. (3.6):

S BI (t) = e − BI t (3.6)

where BI is the failure rate of the basic insulation. The related risk is
indicated as r BI (t).

3.6 Safety and Risk of Class 0 Equipment
Let us consider a Class 0 piece of equipment, that is, an electrical item
with basic insulation in an enclosure without a bonding terminal. 5
If the basic insulation fails and persons touch the faulted enclo-
sure, indirect contact would occur. The safety S 0 of this configuration
coincides with the previous one examined in Sec. 3.4. In fact, the failure
rate of the basic insulation does not change, and therefore

S 0 (t) = S BI (t) (3.7)

The fault potential, though, appears over the whole metal enclo-
sure, increasing the probability that persons are subject to a touch
potential. This causes k 0 (t) > k BI (t). Hence, the risk of electric shock
caused by indirect contacts becomes greater by adding the enclosure,
even though safety is the same.
In formulas:

r 0 (t) > r BI (t) (3.8)

In ordinary locations, the risk r 0 (t) is not considered acceptable by
any standards or codes, which require Class I equipment (i.e., outfitted
with bonding terminals) to be used. 6