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#47455 - 01/17/05 03:27 PM Testing Residential Circuits  
Dave55  Offline
Joined: May 2004
Posts: 697
Crystal Lake, Illinois, USA
I just got the word that Ideal is having problems with production of it's circuit testers/analyzers, which I had ordered. I can test voltage and polarity with other testers I have, but I really wanted the ability to load test. Any ideas on something (small) I can make to load test 120 and 240 volts circuits. If possible I'd like to see what amperage the breakers trip at. Something more professional than heaters and toasters would be nice.


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#47456 - 01/17/05 04:57 PM Re: Testing Residential Circuits  
dereckbc  Offline
Joined: Oct 2003
Posts: 156
Tulsa, OK
Dave I do not know if it is possible to get all those functions in a compact box without electronic control circuits. The problem is heat dissipation. A small box would get too hot too fast. In college for my senior project I made something very similar to the Sure Test meter using momentary switches, resistors, and banana test plugs for meter in a utility box (wished I had patented it). I also used a load box to test higher voltages like 240, but at the time I was an intern at a utility company so I was able to use precision “Ohm Spun” resistors from junk equipment. It would be very expensive to buy them on the market.

In order to test CB trip values you need to get resistance down to 1-ohm @ 100 W to test a 20-amp breaker. Granted a 100-watt resistor is not enough wattage for continuos load, but should work for instantaneous trip region of a 20-amp breaker if using a momentary switch rated @ 100-amps.

Another thought would be a bank of light bulbs, but that is not small. Which brings me back to the point I have been rambling on about. I do not think you can cram 10-pounds of poop in a 1-pound box.

#47457 - 01/17/05 05:04 PM Re: Testing Residential Circuits  
dereckbc  Offline
Joined: Oct 2003
Posts: 156
Tulsa, OK
Dave, one more thought, but it is not small and is ugly. To test breakers you might already have the equipment on the truck. A roll of 12 AWG wire. Figure out how many feet of wire it would take to make one ohm. Walla, a 1-ohm, high wattage, wire wound resistor.

#47458 - 01/17/05 07:19 PM Re: Testing Residential Circuits  
Dave55  Offline
Joined: May 2004
Posts: 697
Crystal Lake, Illinois, USA
Thanks guys, I appreciate the help. Would it simplify the problem if I made something only for 15 and/or 20-amp circuits at 120-volts? Also, it's been way too long since electronics classes. Wouldn't a 1 ohm resistor allow 120-amps? ...and at what current should breakers be tested?


#47459 - 01/17/05 07:25 PM Re: Testing Residential Circuits  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,211
SI,New Zealand
One thing you have to be aware of, is the power dissipation at such a low resistance in the circuit.
With 120V flowing through your 1 ohm resistor, you are dissipating 14.4kW of heat.
Question is, are breakers required to be tested to this degree?.
[Linked Image]
{Message edited to fix up formula}

[This message has been edited by Trumpy (edited 01-17-2005).]

Let's face it, these days if you're not young, you're old - Red Green grin

#47460 - 01/17/05 07:33 PM Re: Testing Residential Circuits  

The wound romex resistor idea reminds me of firing up a PVC heater off of a temp service. Did not want to cut a piece so I pulled one end and tied it to the temp. pulled the other and went to the heater. Left the coil on the ground. Long story short, I came back to a coil of smoking melted plastic.

#47461 - 01/17/05 08:09 PM Re: Testing Residential Circuits  
Dave55  Offline
Joined: May 2004
Posts: 697
Crystal Lake, Illinois, USA
I feel a bit dumb about this, but I'll continue. At 120-volts I want to load the breaker past it's rating to test that it trips. I set up 300-watt bulbs with 5 on the first switch.

1500W/120V=12.5A. Then switch on another 300-watt bulb for 1800W/120V=15A. Then I continue throwing switches until the breaker trips. Is this an effective way to load test? With inverse-time breakers what should the load be? ...and most of all...Is there something better than light bulbs???


#47462 - 01/17/05 09:03 PM Re: Testing Residential Circuits  
dereckbc  Offline
Joined: Oct 2003
Posts: 156
Tulsa, OK
Dave, when I was in the business of commissioning electrical distribution systems we tested at the instantaneous trip level of a breaker, 1-second or less. Generally speaking that level is at 6 times the OCPD rating. It varies, so it is not gospel. The one ohm I quoted is for a 20-amp breaker @ 120 volts. We used custom made load boxes with electronic timers, and switch selectable load resistors. It is not practical for residential and most commercial jobs. That is why I related to my college and poor man hybrids.

Trumpy is right, heat dissipation is a factor. However if you can control the duty cycle, you can use say something like a 1-ohm resistor @ 100 watts. For a poor mans version I thought of using #12 AWG wire. Do not know what the wattage rating is, but I think it is safe to say it is over 100 Watts, but less than 14KW. Duty cycle is the key.

You can certainly use light bulbs; they are cheap and can safely dissipate the heat safely without regard to duty cycle. How many depends on the breakers characteristics and what amount of time you would want the breaker to operate.

If it were me I would wait until the sure test meters are available again. Just too much work for something that doesn’t amount too much more than a toy, and an ugly one at that. However play with it on paper and do some of the math, you would certainly learn from the experience.

#47463 - 01/17/05 09:12 PM Re: Testing Residential Circuits  
wa2ise  Offline
Joined: Nov 2002
Posts: 782
Oradell NJ USA
A roll of 12 AWG wire. Figure out how many feet of wire it would take to make one ohm.

Be aware that you'll get a significant magnetic field when passing 120 amps thru that coil of wire. Things like this were used to demagnetize color TV sets back in the early days of vacuum tube color TVs. You could wipe your credit and ATM cards with it, but there would be more important safety issues that I'd want to be well away from the coil of wire anyway.

#47464 - 01/17/05 09:16 PM Re: Testing Residential Circuits  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Dave I have to ask why would you want to test this?

Personally I feel it is a bad idea to try to test the breakers trip level at the far end of the branch circuit.

If you want to test the breaker do it at the breaker.

You also need to know the trip curves of the breaker, a 20 amp breaker may run for days with 25 amps of load applied.

Remember that dereckbc was talking about testing the instantaneous trip at about 100 amps for a 20 amp breaker. At that level it trips right away without heating the branch circuit wiring.

It seems you want to test the thermal trip which may stress the insulation of the branch circuit wiring.

I do not believe the ideal circuit analyzer tests the trip level of breakers.

Bob Badger
Construction & Maintenance Electrician

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