Re: NFPA petition decision on 2014 NEC 445.20, for portable generators <15kW
In an IATSE Local 481 workshop that I give, we simulate a double fault situation (like what would happen if a defective cord was dragged through the mud) with a unbonded Honda EU6500 to see what would happen.
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We first create a fault in the Hot by attaching a jumper cable from the Hot pocket of a little patch box I made up to the input of a Fault Simulator that basically consists of a variable 16-25k Ohm resister. So that we can obtain precise measurements of leakage current, we attach a second jumper from the output of the Fault Simulator to the input of a Fluke 1587 Insulation Multimeter, and then a third jumper cable from the output of the Fluke 1587 to one of two ground rods we drive.
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To create a second fault we jumper from the neutral pocket of the patch box to the input of a box with just a switch and another jumper from the output of the switch box to another ground rod. We start by opening the switch on the switch box and closing the Fault Simulator (maximum resistance) so that we can regulate the current leaking to earth.
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When we fired up the generator and supply power to our fault circuit, we see immediately on the Fluke 1587 that there is 4.9mAmps leaking to earth. Clamping a Megger DCM300E Leakage meter onto the jumper going from the Neutral pocket of the patch box to the second ground rod, we see that the Fault Current is returning to the generator's windings through the second fault that we established on the Neutral as depicted in the illustration below (click on the image to enlarge it, then hit the back button to return to the forum.)
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As we gradually reduce the resistance of the Fault Simulator by turning its' rheostat, the leakage current begins to rise. When the Fault Simulator is all the way open (minimum resistance) we get a clear Ground Fault of 7.9mAmps (according to the readout of the Fluke) and the breaker does not trip. Closing the switch on the Switch Box so that there is only one fault in our system, the current leakage to earth stops. Opening the switch on the switch box, the leak begins again.
The purpose of this demonstration is to show that a double fault - one in the Hot, and a second in the Neutral - creates a potentially hazardous situation because a path (circuit) now exists for fault current to return to the generator windings even though there is no neutral to ground bond. If an individual comes into contact with this ground fault circuit created by the two faults, fault current will travel through the individual on its' way back to the generator's windings. And, as demonstrated if either of the faults is of a high resistance, the current will not be high enough to trip the breaker, and the individual will receive a sustained shock that can be potentially fatal. The Figure below illustrates why an individual receives a shock when there are two faults in a distribution system.
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The reason NEC 445.20 is so adamant about there being GFCI protection when a un-bonded generator supplies 240V is that double Faults with Floating Neutrals are especially hazardous. Without a Neutral/Ground bond, as illustrated below, a double fault condition like that illustrated below can expose an individual touching faulty equipment to 240 volt potential.
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What I find particularly scary about the double fault situation in my workshop demonstration is that a GFCI plugged into the 120V receptacle of the generator offers no protection since, as the illustration above clearly depicts, the fault current returns to the neutral via the second fault before passing back through the GFCI Current Transformer.
Is the Ground Fault current of 7.9mAmps in this demonstration a hazard? It is on the threshold of being painful but not potentially fatal because an individual experiencing 7.9mAmps still has muscular control and will be able to remove them selves from the source of the shock. But, will an individual coming into contact with this ground fault receive 7.9mAmps? All the information you need is given here. If you don’t know how to answer this question I would suggest you use this link to read the summary of my IA workshop because someone coming between the double faults in that situation would be electrocuted without the GFCI even sensing a leak of any kind.
Guy Holt, Gaffer
ScreenLight & Grip
rentals@screenlightandgrip.com
