"As I mused, the fire burned"

Reflection on life as a person of faith.

The Systems Approach to Forensic Investigation

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I like to say that I practice forensic electrical engineering as a hobby, working as a contract specialist for a local forensic engineering firm.  It fulfills my love of figuring out how things work, solving puzzles, inductive and deductive reasoning and helps keep my engineering license current as a bonus.

Most forensic investigations can be solved with what you might call first-level reasoning.  In a fire investigation, for example, you can use your knowledge of fire dynamics and fire chemistry to track the progress of the fire back to a likely area or point of origin.  Then you sift through the debris in the suspected area of origin to attempt to find an energy source.  Once you’ve located all the potential energy sources, you establish a series of hypothesis and test these against the evidence.  This leaves you with a predominant cause, with other less probable causes.  Further examination of the suspected causes will remove or confirm the remaining hypothesis.

As soon as your suspected cause includes an electrical source, this sometimes requires moving beyond first-level reasoning.  This is where a systems approach to investigation is important.

Sometimes you may identify an obvious electrical cause – a poor electrical connection that shows signs of heat damage in excess of the remainder of the fire, for example.  At that point, you may stop the electrical inquiry.  However, is an obvious cause does not present itself, but there is evidence of a possible electrical cause, the investigator has to take a systems approach.

The electrical wiring of a building always has to be viewed from a systems perspective.  An electrical engineer would call building wiring an electrical network, with sources, loads, and non-linear circuit components.  You don’t need to have the electrical lingo down to use a systems approach, which is the real benefit of a systems approach to investigation.  Any reasonably inquisitive person can carry through the process.

For example, the possible cause electrical device you are examining, is connected to a branch circuit which usually feeds a number of other devices.  In a home, almost every branch circuit will include lighting and  electrical outlets (with a few code-driven exceptions such as appliance circuits).  As the electrical current flows through that branch circuit, every attached device has the possibility of influencing the overall characteristics of the system.

Let’s say you find a short-circuit wired connection that is dead-centre in the area of origin, and the wiring displays characteristic signs of very high temperatures.  The first troubling question is why the over-current protective device, the circuit breaker, did not function to remove power?  Circuit breakers do a rather poor job protecting against anything but direct shorts, but in the case of a direct short a circuit breaker usually operates quite quickly.  So why did the circuit breaker not operate to clear the fault?

Your first urge will be to examine the circuit breaker which is a good urge, but it is important to consider the entire network between the service entrance and the fire area, including each of the devices along the way.  Anything interconnected with the branch circuit has the potential of limiting the flow of current to a point where the circuit breaker will not function quickly enough to clear the fault.

Ohm’s law gives a good first-order approximation for residential circuits: V=IR  (voltage equals current times resistance).

In a 120 volt circuit, if you have a dead short (say of 0.01 ohms), the current gets quite large. 120 V = I * (0.01 ohms)       I = 12,000 amperes  Which would cause a functional breaker to immediately operate.

In reality the wiring itself presents a resistive load to the branch circuit of perhaps a few ohms.  That needs to be considered in longer runs of wiring, or if the wiring was not done to code.  I’ll just ignore that at this point.

Now, assume that one of the electrical outlets installed along the branch circuit has a poor connection that is heating, but is not hot enough to cause a problem.  This can sometimes be caused by inexpensive outlets (bought at big-box stores) in work done by non-electricians, that do not use wire screws to attach the wires to the outlet, but rather use a ‘back stab’ connector that uses a spring to hold the wire in place.  These types of outlets should be banned outright, as they are too often causal in electrical problems.

If you insert the wire into the outlet, and then apply any force to the wire (like when pushing the outlet into the box) the wire will twist under the spring, which can impair the connection.  How much can the connection be impaired before it is a problem?  Considering my example of the short-circuit, and add in a series fault that adds a resistance of 5 ohms to the circuit (very small).

Now the total resistance seen by the fault current flow is 5.01 ohms, still quite small.  Look what happens to the current: V = IR, 120 V = I * (5.01 ohms),  I = 24 amperes

Now, even on a 15 amp circuit, that current will take some time to heat the breaker up to the point where it will trip.  Time that is measured in minutes, and not seconds.  Circuit breakers are really good at clearing high-current faults very quickly, but faults like this can last for a long period.  Long enough to start a fire.  (this is a very low power heat source, but it is easy to demonstrate higher powers in similar circumstances).

So, taking a systems approach requires that you examine every device between the service entrance and the suspected cause.  It also includes performing measurements whenever possible – what is the resistance between the service entrance and this device’s terminals?  What is the overall resistance of the circuit from the source (the breaker) to the end of the undamaged run of wiring?  (assuming the power is off, you short circuit the black and white wires at the last point of undamaged wire run, remove the breaker and carry out a resistance measurement between the black wire on the breaker, and the neutral bus.  You could also jumper to two sides of an electrical outlet together).

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Written by sameo416

June 29, 2013 at 10:16 am

Posted in Uncategorized

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