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Low Earth Orbit
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Why An Open Neutral Kills 120V Devices
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<blockquote data-quote="Langston Holland" data-source="post: 58710" data-attributes="member: 171"><p>Re: Why An Open Neutral Kills 120V Devices</p><p></p><p><u>Just to confirm what many folks figured out already about what turned out to be a non-profit gig</u>:</p><p></p><p>1. The genny was fine.</p><p>2. The genny didn't have its neutral and ground bonded.</p><p>3. The genny was switched correctly to 120/208V 3-phase.</p><p>4. The hots were connected to the genny correctly.</p><p>5. Our neutral cable was connected to the genny ground and our ground cable was connected to the genny neutral.</p><p></p><p><u>A take home lesson</u>:</p><p></p><p>Metering the genny with a high quality multimeter prior to connecting our cables didn't reveal the not-so-obvious problem. Metering between the (3) hot lugs and the neutral lug showed a solid 120V. Metering between the (3) hot lugs and the ground lug very probably showed a near 120V that varied by several volts over time. It's also likely that the meter probes were applied for too short a time to see this variation.</p><p></p><p>The reason a high impedance multimeter can read what appears to be close to normal voltage between hot and an unbonded ground is due to the capacitance between that ground and nearby neutral. Thus what is needed for proper electrical source measurements is a load - but not your sound system! <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":)" title="Smile :)" data-smilie="1"data-shortname=":)" /> A cheap electric trouble light or heater or the like will zero that capacitive voltage reading. A fancy way to apply a load is using a circuit analyzer such as the one I'm giving away to the first guy that answers the questions correctly (see my next post). These circuit analyzers apply a 15 to 20 amp resistive load on the circuit for 1/2 to 1 cycle of the 50Hz or 60Hz sine. This allows them to use a low wattage resistor and have the device ready (cooled down) for another measurement after 20 seconds.</p><p></p><p><u>The problem with using an electrical source without neutral and ground bonding</u>:</p><p></p><p>If you connect the cables correctly, everything should work properly, thus you may think all is well. I wonder how many times we've used gennys without neutral/ground bonding successfully and not known it? Regardless, it won't happen to me again. How about you?</p><p></p><p>If a powered piece of equipment malfunctions with the hot connecting to chassis there is no return path through the ground wire to trip the breaker, thus the chassis remains electrified. This obviously is potentially deadly to anyone that touches the malfunctioning component. The component may still work fine with a hot chassis.</p><p></p><p>If this happens to be a genny gig and the genny doesn't have continuity with the ground due to its rubber tires and the trailer tongue is insulated from the ground in some fashion, the electrified chassis may not shock the person touching it. But don't worry - you can still die - what is required is completing the circuit with your body by touching the hot chassis and another component with an issue.</p></blockquote><p></p>
[QUOTE="Langston Holland, post: 58710, member: 171"] Re: Why An Open Neutral Kills 120V Devices [u]Just to confirm what many folks figured out already about what turned out to be a non-profit gig[/u]: 1. The genny was fine. 2. The genny didn't have its neutral and ground bonded. 3. The genny was switched correctly to 120/208V 3-phase. 4. The hots were connected to the genny correctly. 5. Our neutral cable was connected to the genny ground and our ground cable was connected to the genny neutral. [u]A take home lesson[/u]: Metering the genny with a high quality multimeter prior to connecting our cables didn't reveal the not-so-obvious problem. Metering between the (3) hot lugs and the neutral lug showed a solid 120V. Metering between the (3) hot lugs and the ground lug very probably showed a near 120V that varied by several volts over time. It's also likely that the meter probes were applied for too short a time to see this variation. The reason a high impedance multimeter can read what appears to be close to normal voltage between hot and an unbonded ground is due to the capacitance between that ground and nearby neutral. Thus what is needed for proper electrical source measurements is a load - but not your sound system! :) A cheap electric trouble light or heater or the like will zero that capacitive voltage reading. A fancy way to apply a load is using a circuit analyzer such as the one I'm giving away to the first guy that answers the questions correctly (see my next post). These circuit analyzers apply a 15 to 20 amp resistive load on the circuit for 1/2 to 1 cycle of the 50Hz or 60Hz sine. This allows them to use a low wattage resistor and have the device ready (cooled down) for another measurement after 20 seconds. [u]The problem with using an electrical source without neutral and ground bonding[/u]: If you connect the cables correctly, everything should work properly, thus you may think all is well. I wonder how many times we've used gennys without neutral/ground bonding successfully and not known it? Regardless, it won't happen to me again. How about you? If a powered piece of equipment malfunctions with the hot connecting to chassis there is no return path through the ground wire to trip the breaker, thus the chassis remains electrified. This obviously is potentially deadly to anyone that touches the malfunctioning component. The component may still work fine with a hot chassis. If this happens to be a genny gig and the genny doesn't have continuity with the ground due to its rubber tires and the trailer tongue is insulated from the ground in some fashion, the electrified chassis may not shock the person touching it. But don't worry - you can still die - what is required is completing the circuit with your body by touching the hot chassis and another component with an issue. [/QUOTE]
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