Wednesday, December 8, 2021

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RVelectricity™ – Everything you know is wrong (about circuit breakers)

Dear Readers,
The amount of misinformation posted on Facebook is legendary, so please don’t take advice from any FB “electrical expert.” It seems a bit like the original Firesign Theatre album, “Everything You Know Is Wrong”!

You should be aware that all of my RVelectricity articles on RVtravel.com and my own RVelectricity Facebook group are fully peer-reviewed by my own administrators and moderators – who are all highly skilled in electricity and code. Nope, I can’t get away with publishing any incorrect information, so you can trust what we post.

Here’s what started this thread on a Facebook Group (not mine)…

I want to convert my 30 amp motorhome to 50 amp. Can I just put in a larger circuit breaker as someone said to do? 

One of the answers (that’s wrong)

A good 30-amp circuit breaker takes 6.5 hours at 50-amp flow to trip. And yes, the larger breaker can source even more. If he is tripping the breaker and wants it fixed that would be a different issue. (WRONG)

Another answer (that’s also wrong)

A 30-amp circuit breaker will trip instantly as soon as it goes over exactly 30 amperes (WRONG)

So, what’s right?

First of all, NEVER put in a higher-amperage circuit breaker than the original one that the wiring will support. So no swapping in a 50 amp breaker for a 30-amp one. That’s a recipe for disaster!

Circuit breakers that you would find in your residential, campground or RV travel are all thermal/magnetic overcurrent protection devices. That is, the breakers are really two protection devices in one.

Divide and conquer

Inside of these breakers are two different ways that they can “trip” or “clear the fault” – as it’s called in the trade.

First, there’s a bimetal strip that gets hot if you draw more current than the breaker is rated for (up to 5x the breaker rating). When it gets hot enough from a sustained overload, it causes the spring to trip a lever and open up the contact.

This is what allows the circuit breaker to provide you with enough power for momentary current surges like what’s required by an air conditioner compressor starting up. For example, above is one of my High Rate Data Logger (HRDL, like in “turtle”) curves for reference that shows just how much extra surge current is needed to start an air conditioner with the stock starting capacitor as well as the SoftStartRV controller. Without a thermal delay, any normal air conditioner would trip a 30-amp circuit breaker every time the compressor started up.

How long does a breaker take to trip with a moderate overload?

Actually, it takes a lot longer to trip than one might think…. Let’s look at a standard Class B OCPD (overcurrent protection device) Trip Curve for example.

The bottom axis show you the amount of current in multiples of the rated amperage. So, for a 30-amp breaker, 1 would show you the trip time at 30 amps, 2 would show you the time at 60 amps, and 3 would show you the time at 90 amps.

The vertical axis shows you how long the time window is in seconds. So 60 amps of current through a 30-amp breaker should trip in between 5 and 20 seconds. At 1.5 times the rated 30 amps of current (45 amp overload), that same breaker could take anywhere from 30 to 200 seconds to trip.

But wait, there’s more!

Take a look at my chart showing how quickly a circuit breaker will clear a short circuit. I gathered this data myself using HRDL running at 48,000 times per second and 24-bits resolution. I use my own diagrams from my own lab data whenever possible. 

Once you get above 4 times the rated current (in this case 120 amps from a dead short-circuit), the magnetic tripping part of the circuit breaker kicks in (literally). There’s an electromagnet that slaps a little hammer against the latch, causing it to open very rapidly. So, if you look at the curve above the number 4, you’ll see that this circuit breaker could “clear the fault” in anywhere from 0.002 seconds (2 milliseconds) to around 1 second. This is the big spark you see when you accidentally short out a wire with a screwdriver.

Here’s what this looks like when you short out a 100-foot length of 10-gauge copper fed by a standard 30-amp circuit breaker. Yes, that’s me in my lab doing an experiment to demonstrate just how fast circuit breakers trip on a bolted short circuit. And, yes, it should be faster than the blink of an eye at maybe 15 milliseconds (0.015 seconds) or so.

What does this all mean?

Well, circuit breakers are there to protect your wiring, not your heart. Their job is to prevent either a moderate overcurrent from heating up the wiring enough to start a fire, or clearing a short circuit, which could dump hundreds of amps through a wire and cause it to melt or even vaporize. GFCIs (ground-fault circuit interrupters) are there to protect you (and your heart) from more than the 5mA of current that’s needed to trip them. But that’s a different story altogether.

And, kids, DON’T try this at home…

I’m doing these short-circuit demonstrations in the safest way possible, and I have 5 decades of experience to help keep me safe. So please, DO NOT try to replicate any of my experiments on your own. Yes, these are very dangerous.

If you want to take a really deep dive into how to interpret trip curves for Overcurrent Protection Devices, here’s a great resource: StudyElectrical.com.

Let’s play safe out there….

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.

##RVT1023

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Tim Burke
15 days ago

Mike, I know that I am late to this party, but I am surprised that in your snapshot, i didn’t see either HV Kleins or at least class 00 HV Gloves.

Mike Fields
1 month ago

In your brief mention of a GFCI breaker protecting your heart, it is important to understand that the GFCI ONLY protects you (and your heart) when it is a leakage path to ground (unequal current in hot/neutral). If you get across the hot and neutral lines, the GFCI does NOT offer the protection. A subtle but very important difference

jon schacher
1 month ago

Your explanation of how the circuit breaker works is good reading. As an addendum to your article, suggest adding that the circuit breaker protects the wiring downstream from it to the next over current protection device (if any) such as in the distribution panel. The current rating of that wiring is per NEC (or similar) specifications for the size of conductor, length of run, and type of insulation material used to encase the conductor. Those “comments” you led off with in the article are good examples of knowing just enough to be dangerous.
The role of the distribution panel and conductors might be a good subject for your next article.

Dick and Sandy near Buffalo, NY
1 month ago

Great article…keep them coming. Stay safe, Stay well.

Roger Marble
1 month ago

Good stuff. It must be nice to have a “Laboratory” where you can run your own tests to collect data to share with your readers. Tire testing is a lot more expensive with used test drums starting at $50,000 to $200,000.

Mike Sokol
1 month ago
Reply to  Roger Marble

My meters are only a few thousand bucks each, but it does add up…😁

Mike Sokol
1 month ago
Reply to  Roger Marble

BTW: Since my lab is located just outside of Funkstown, and since I can’t use the trademarked name Skunkworks, I call it the Funkworks! 😁

Last edited 1 month ago by Mike Sokol
Vic Whitmore
1 month ago

Mike,
Thanks so much for the detailed explanation of a circuit breaker. The graph is very revealing and the two different trip mechanisms are crucial to understanding real world operation.
Old guys like me keep learning new tricks.

Mike Sokol
1 month ago
Reply to  Vic Whitmore

You’re very welcome. A thermal magnetic breaker acts just like a slow-blow fuse. A really cool design.

J J
1 month ago
Reply to  Mike Sokol

One of my longest careers was in corporate jet aircraft avionics and electrical maintenance. Every breaker was solely magnetic, no thermal component at all. You can’t just pull over to the side of the airway if there is a fire and “bailing out” in the event of a fire would literally require bailing out. 🙂

Steve Comstock
1 month ago

“Waiting for the Electrician or Someone Like Him”
Since you mentioned Firesign Theater, seems appropriate, no? 😉

Ron T.
1 month ago
Reply to  Mike Sokol

Steve stole my thunder, but I also have their decidedly un-pc “Don’t Crush That Dwarf Hand Me the Pliers”

Mike Sokol
1 month ago
Reply to  Ron T.

Zeno’s Paradox…
“Antelope Freeway, 1 mile.”
“Antelope Freeway, 1/2 mile.”
“Antelope Freeway, 1/4 mile.”
“Antelope Freeway, 1/8 mile.”
“Antelope Freeway, 1/16 mile.”
“Antelope Freeway, 1/32 mile.”
“Antelope Freeway, 1/64 mile.”

Admin
RV Staff(@rvstaff)
1 month ago
Reply to  Mike Sokol

Talk about a dichotomy. See? I learn something new from you every day, Mike. Thanks! Have a good night. 🙂 (And I’ll be looking for your JAM Session in the morning. 😆 ) –Diane

Ron Seidl
1 month ago

Mike, Not only are your answers reviewed before you publish them, but countless other knowledgeable individuals see them after you publish.
Ron Seidl P.E.

Dan
1 month ago

Reminds me of people putting pennies in the screw-in fuses in houses, or wrapping old style automotive tube fuses in foil. I have to admit I have done that just to get home. The blown fuse is trying to tell you that something on that circuit is failing. Fix that first.

J J
1 month ago

This is a great statement: “… circuit breakers are there to protect your wiring …”

Way too many people think a fuse or a breaker is there to protect the device it’s connected to. It’s not. Why?

Because of the concept of a “series circuit”. A two wire power cable is actually a series circuit. In a series circuit, ALL CURENT IS EQUAL.

Before a fuse can blow or a breaker can trip, the high current (amps) needed to blow the fuse or trip a breaker has already gone all the way through the device and “come out” the ground wire for DC or the neutral wire for AC, so to speak.

Yes, “What goes in must come out” also applies to electricity.

So a fuse or a breaker is circuit protective device. While it may help protect the device on the other end by cutting off the power before further damage occurs to the defective device, that’s not always the case and is incidental to its primary purpose: protecting the source of power in order to avoid a fire.