Introduction
We have been told that the current takes the path of least resistance.
Wrong!
Actually, it’s worse than wrong! It’s a dangerous way to think about electricity and the flow of current.
When I was teaching, my students would go out of their way to say, “current takes the path of least resistance.” They knew that I was sure to quickly respond with a scolding side lecture (yes, it is a pet peeve – and they knew it!). My revulsion to this myth reflects the painful electrical shocks I’ve received. One incident occurred when I was a young man. I’ll never forget the electrifying pain associated with stripping wax from a floor while standing in a pool of wax remover. Something broke inside the machine. Believe you me, I knew it was electrical!
Safety ground
To understand why, consider the power connection as shown in Figure 1. This B&K Precision 1550 power supply features a metal chassis. The image shows the IEC power connection as it enters the enclosure. For code compliance, the ground wire is securely bonded to the metal enclosure (chassis). This low-resistance connection is soldered and features an internal-tooth star washer.
Figure 1: IEC connection for a metal chassis power supply featuring a solid ground. This is an essential code-compliant aspect of electrical safety.
Tech Tip: The IEC connector in Figure 1 has an integral fuse. For convenience, the user may replace the fuse without opening the equipment.
Worst-case scenario
While it’s extremely unlikely, let’s assume a worst-case scenario:
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Imagine a nefarious gremlin has entered the enclosure and connected the hot wire directly to the chassis.
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Let’s also assume that the gremlin has replaced the power supply’s fuse with a copper slug.
In this scenario the metal enclosure is electrically hot, and the power supply is unable to disconnect from the AC mains.
Returning to our “Path of Least Resistance” myth, suppose you’re barefoot in a damp basement, and you touch the metal chassis of the power supply.
Stop and think about this.
There are now two current paths that are vitally important to us including:
- the path from the chassis to ground via the 3rd wire safety ground.
- the path from the chassis through your wet feet to the building’s earth reference.
If our myth was true, we would only worry about the first path. Assuming your workbench and shop are wired correctly and that you have not removed the ground lug from power cord.
If our myth is false — as it is — we need to consider the current flowing through your body as shown in Figure 2.
Figure 2: Simplified circuit showing the two circuit paths. Resistance values are for demonstration purpose only.
CAUTION: Figure 2 presents idealized values for wire and body resistance. Always consult a certified professional to verify mains wiring is to code. Also, consult a professional to verify that your non-insulated chassis is grounded according the international, federal, and state code. It is illegal for non-certified personnel to perform this type of wiring in some jurisdictions.
Current takes all paths
Using Ohm’s law, we see a major problem in Figure 2. If you are lucky, the associated line fuse or circuit breaker will open. But, if our gremlin was also in the fuse box, a sustained 60 A current will flow from hot to ground. This causes a 60 mA current flow through your body. This is serious, as 60 mA is above the let-go threshold.
There is a big difference between 2 Ω and 1 kΩ. It’s tempting to say current will flow through the path of least resistance. However, we can see that this was an absurd and very dangerous assumption.
Never defeat the safety ground on electrical equipment. Also, please take rudimentary safety precautions like using a ground fault circuit interrupter in damp locations and electrically insulating your workbench.
Oh, and avoid standing in water while operating electrical equipment, especially old equipment like that floor buffer designed in an era before sensible safety codes.
Stay safe!
Aaron
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.