Rug Doctors and "smart" QO breakers

[ModbusMan]

So either way, something is going to have to be taken apart to do this. Unless I'm missing something.

Indeed. Thought just hit me that sitting in my drawer at the office (remember those?) is a power strip where I removed roughly 3in of the power cord's outer jacket just above the multi-tap so that I could amp clamp various pieces of gear that I was testing, or to test small split-core CTs I suspected of malfunctioning with a known load value (how I miss incandescent light bulbs). Next time I rent one of these, I can run in and snag that, then clamp while it's running and see where the power's flowing.

 

[OldSparks]

current is same at every point in the circuit.

That's simply not true. GFCI's work by virtue of the fact that current on the "hot" is approximately equal to or greater than 7ma more than on the neutral, making the assumption that the extra current is travelling through a human.

 

[Barbqranch]

Can't you test for continuity between ground and neutral, and also turn on the switch and check for conduction between hot and neutral, but not between hot and ground?

 

[busman]

current is same at every point in the circuit.

What you likely had happening and is pretty common particularly with standard QO 15 and 20 amp single pole breakers is they have a lower magnetic trip setting than competitor versions of these breakers. Not sure whether their AFCI and GFCI versions have lower mag trip or not, seems likely they do not but I don't know for certain. When you plug it in via extension cord and it now will start without tripping, you have added enough circuit resistance to limit the starting current enough to not get into magnetic trip range. Square D does have single pole 15 and 20 amp standard breakers with a HM (high magnetic) suffix in the catalog number that have a higher magnetic trip setting and use of those also will solve this problem.

That is not entirely true. Capacitance will cause some neutral current to flow on the EGC that is greater than there would be with a shorter cord.

Mark

 

[retirede]

You guys are misconstruing Kwired’s point. He is basically stating Kirchoff’s law in a simplistic way.

He was countering the idea that when current starts to flow in a circuit, that the current at one end of the wire is greater than it is at the other end, and eventually evens out.

 

[Paul_in_Berkeley]

You guys are misconstruing Kwired’s point. He is basically stating Kirchoff’s law in a simplistic way.

He was countering the idea that when current starts to flow in a circuit, that the current at one end of the wire is greater than it is at the other end, and eventually evens out.

At the speed of light, it evens pretty damn quick!

Paul

 

[winnie]

Kirchoff's law holds; the exact same current flows everywhere in the circuit, no evening out required.

However once you start to consider speed of light effects, your concept of 'circuit' has to become rather expansive.

If you have a long two conductor cable, shorted at one end, and connected to a supply at the other end. Apply voltage at the supply end current will _immediately_ start to flow at the supply end. But current is not immediately flowing through the short at the far end of the cable (speed of light and all). So you have this loop of wire, with different current flow in different parts of the loop. You must include the 'current' flowing into charging the electromagnetic field surrounding the wires. The 'circuit' becomes multiple circuit paths and if you only look at a subset of those multiple paths you will likely see imbalances.

You won't have more current flowing into the hot than the neutral unless the two have vastly different capacitance to the surroundings, which is another way of saying unbalanced current to _ground_ via capacitive coupling.

 

[Little Bill]

Can't you test for continuity between ground and neutral, and also turn on the switch and check for conduction between hot and neutral, but not between hot and ground?

You won't have continuity (or you shouldn't) between neutral and ground on the machine's plug. You would have it with the machine plugged in as the neutral and ground would be bonded at the panel. But you wouldn't check continuity with it plugged in.

I just checked an electric drill plug for continuity between hot and neutral while pulling the start trigger. It, of course, showed some continuity, but doesn't tell you which is hot and which is neutral. No continuity shown on my meter, neutral to ground, with trigger pulled, as expected.

 

[wwhitney]

I just checked an electric drill plug for continuity between hot and neutral while pulling the start trigger. It, of course, showed some continuity, but doesn't tell you which is hot and which is neutral. No continuity shown on my meter, neutral to ground, with trigger pulled, as expected.

So for such an electric drill, if the plug was replaced by someone else, and you are worried that the N/G pins on the plug were reversed, you could pull the trigger and check for continuity H-N vs H-G. If the pins are reversed, you'd see continuity H-G rather than H-N.

Seems to me the most likely explanation for the OP's experience is that the machine has an internal high impedance ground fault. Passing enough current to trip a GFCI, but not enough to trip a regular breaker.

Cheers, Wayne

 

[Little Bill]

So for such an electric drill, if the plug was replaced by someone else, and you are worried that the N/G pins on the plug were reversed, you could pull the trigger and check for continuity H-N vs H-G. If the pins are reversed, you'd see continuity H-G rather than H-N.

It's been a few days since I read the OP. I was thinking he was concerned with the H/N being reversed. But I went back and looked and he was concerned with N/G being reversed. My answer, and test, was based on H/N.
It wouldn't be a normal thing I would think of to have the N/G pins reversed on a plug. It could happen, but not something that I have ever ran into.

 

[kwired]

You guys are misconstruing Kwired’s point. He is basically stating Kirchoff’s law in a simplistic way.

He was countering the idea that when current starts to flow in a circuit, that the current at one end of the wire is greater than it is at the other end, and eventually evens out.

Exactly.

Yes there can be leakage that will imbalance current and trip a GFCI, but if you have leakage (beyond negligible levels) you have more than one current path and Kirchoff laws still apply - whatever enters one node also leaves that node. The leakage just happens to be an unacceptable path to the GFCI and that causes the trip. If there is only one path current is equal at every point in that entire path.

 

[kwired]

You won't have continuity (or you shouldn't) between neutral and ground on the machine's plug. You would have it with the machine plugged in as the neutral and ground would be bonded at the panel. But you wouldn't check continuity with it plugged in.

I just checked an electric drill plug for continuity between hot and neutral while pulling the start trigger. It, of course, showed some continuity, but doesn't tell you which is hot and which is neutral. No continuity shown on my meter, neutral to ground, with trigger pulled, as expected.

If someone miswired a replacement cord cap, they could have the neutral pin of the plug connected to a current carrying conductor. As long as the EGC of the cord is not connected to the ungrounded pin the item will still function with no GFCI involved but current is flowing on the EGC of the branch circuit instead of the neutral.

A simple continuity test of the plug pins (unplugged) shouldn't have any continuity between the EGC and either of the other pins or it will likely be problem if a GFCI is involved.