Page 60 - Electrical Safety of Low Voltage Systems

Mathematical Principles of Electrical Safety 43

FAQs
Q. If safety cannot ever be absolute (i.e., 100%), can we design safe electrical
installations?
A. An electrical installation is deemed safe if the residual risk imposed to
persons falls below the acceptable risk. The minimum acceptable risk is estab-
lished, as a “legal” requirement, by local Authority Having Jurisdiction (AHJ),
which can enforce national electrical codes and/or “recommended practice”
as provided by national (BS, CEI, DIN, etc.) and international (e.g., IEC, IEEE)
standards. Thus, the answer is yes, we can design safe electrical installations
within the above assumption.

Q. Can power systems be foolproof?
A. No, they cannot. Erratic human behaviors are difficult to predict and
this makes it impossible to design for any possible scenario. In other words,
should the electrical installations be safe with regard to untrained people?
What amount of “awareness” of the danger of electricity should we assume
for persons?
If this amount is assumed too high (e.g., each person is supposed to pos-
sess knowledge at electrical engineer level), we would have more permissible
design criteria (e.g., live parts might even be exposed, as electrical engineers
have a very high awareness of the danger caused by direct contacts). On the
other hand, if the assumed level of awareness is set too low, we might not be
able to design any installations, as the safety requirements would become too
conservative.
In high-voltage substations, we can reasonably expect trained people, un-
like in dwelling units, and therefore, we should assume two different kinds of
“standard person” in the two realms.
The answer, once again, is to refer to code and standards to understand the
amount of “electrical awareness,” which is considered acceptable, and design
accordingly.

Q. Does the failure of Class II equipment cause direct or indirect contact?
A. Indirect contact is defined as contact with metal parts not normally live. If
double insulated items are in insulating enclosures, electric shock from Class
II equipment can occur only by direct contact. Should Class II equipment
be enclosed in a metal frame, the failure of the double insulation will cause,
instead, indirect contact.
Q. If the failure rate of a circuit breaker is 0.0052 failure per year, how many
devices, among a population of 100, will “survive” after 1 year?
A. In the discrete sense, the failure rate can be expressed as:

1 R i (t) − R i (t + t) 1 100 − R i (1)
i = =
R i (t) t 100 1