Authors: Roy E. Voshall
Suggested Courses: Circuits I
Jane is a recent graduate engineer working for an electrical consulting company (Eleck
Inc.). She is given the job of laying out the wiring for a new home. In particular, she
must specify the circuit breakers that protect the circuits to the wall plugs of each room
and the ceiling lights. Normally these circuits are wired with AWG #14 wire and are
protected by 115 V/ 15 A breakers.
A co-worker, Jerry, who is a mechanical engineer tells her that she should specify 115
V/ 20 A circuit breakers because each circuit then could handle higher current appliances
or more appliances at a time. But the house will be wired with # 14 wire which has a
continuous current rating of 15 A. Jerry's reaction to this is "Oh, forget that the
house will be wired with #14. The overload caused by the appliances will only last a few
minutes and a 20 A breaker will provide adequate protection." Subsequently, the house
was built with the #14 wire installed in the appropriate circuits and protected with 115
V/ 20 A circuit breakers as specified by Jane.
Two months after the house was built and the family moved in, a fire occurred in the
house causing $75,000 worth of damage. The fire marshall's report stated that the fire was
caused by an electric toaster having a short circuit in it. The report also stated that
the short circuit current in the toaster was estimated to be 550 A and lasting for 10
seconds. The important parameters in causing an electrical fire is the energy from the
electrical short and this energy is related to i2t. The i2t for this
short circuit is approximately 3 x 106 amp2-seconds.
(i2t = 5502 x 10 seconds = 3 x 106 amp2-seconds)
The specifications for circuit breakers include:
Overload currents in terms of i2t
Maximum short circuit fault current
For the 15 A breaker, the i2t is 2 x 106 amp2-seconds
and for the 20 A breaker the i2t is 4 x 106 amp2-seconds.
Note that the tripping time of the breaker depends on this i2t value. When the
overload currents are larger, the time to trip the breaker is shorter. Conversely, when
the overload currents are smaller, the time to trip the breaker is longer.
II. Numerical and Ethical Problems
1. Was Jane justified in specifying the 20 A breaker rather than the 15 A breaker for
this house? Explain your reasoning.
2. What are the ethical responsibilities of Jane and Jerry in regard to this accident?
Which parties will be liable in this case? Please refer to the NSPE (or other professional
society) code of ethics for reasons supporting your arguments.
III. Solutions to Numerical and Ethical Problems
The i2t for the toaster short circuit, based on estimates is: i2t=
(550)2(15_= 4.5E6 Ampere2-seconds. Due to the rough estimates involved,
only one significant figure is used, thus 5E6 Ampere2-seconds.
This corresponds (approximately) to the rating of a 20 Ampere breaker. If a 15 Ampere
breaker was used, the i2t rating of 2E6 Ampere2-seconds would have
reduced the burning time of the 550 Ampere short-circuit current to:
For the 20 Ampere breaker we compute:
This is a significant increase in the heating time, and could be the difference between
combustion occurring or not, relative to the study cases.
Note that it may have been the case that the 20 Ampere breaker could have
remained closed for an indefinitely long time. This is so because the i2t
corresponding to the toaster short circuit was estimated to be marginally close, but less
than, the 20 Ampere breaker 12t rating. Because of manufacturing
variability, the true i2t of the breaker is subject to a tolerance, say 10%.
And note that the fire marshal's estimates are necessarily imprecise. Indeed, one should
include some estimate of the probable error, such as "17 seconds 20%."
The shorter heating time attendant to using a 15 Ampere breaker may have prevented a
fire in the toaster and the subsequent house fire.
It was the ethical responsibility of Jane to check the National Electrical Safety Code
(NESC), as well as local building codes, and to determine whether it is justifiable to
specify a 20 A breaker rather than a 15 A breaker
Jane should have investigated the specifications of each breaker and determine whether
there are any problems such as i2t overload. She violated the National Society
of Professional Engineers Code of Ethics for Engineers Fundamental Canon No. 1, and Rules
of Practice No.1, which deal with the safety, health, and welfare of the public.2
Jerry violated the 2nd Canon and the 2nd Rules of Practice which
demands that engineers shall perform services only in the areas of their competence.
It must be noted that Eleck Inc. is ethically responsible, and very likely legally
liable, for the fire. A. Licensed professional electrical engineer must sign and seal
drawings, specifications, etc. having to do with public safety. If Jane was not a P.E.
qualified to do electrical power engineering, a P.E. must supervise and sign/seal the
work. The P.E. responsible for the work would very likely have their P.E. license
suspended, or possibly revoked, because of negligence. Additional penalties may be
administered by the state entity responsible for P.E. licensing.
The mistake is a flagrant one because the NESC Code requirement of a 15 Ampere breaker
is fundamental and well known in the electrical contracting trade, and by electricians and
building inspectors. These individuals also may share culpability here. Although not
stated in the problem, Eleck Inc. would need a contractor's license to perform the
indicated services. Thus, they would be in jeopardy of losing the license, or at least
having it suspended.