Why An Open Neutral Kills 120V Devices

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

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.

Yup.... Simple neon 3-light cube testers are evil. One of the things that got me thinking about RPBG outlet testing failure was that I was designing a 3-light tester using 100 watt bulbs to load the circuits with around 1 amp each. That would remove any high-impedance voltages from capacitive coupling effects that can create false good readings.

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.

Again, exactly correct. I've been experimenting with NCVT (Non Contact Voltage Testers) such as the Fluke VoltAlert (Standard 90 to 1,000 volt model) for use in identifying hot-chassis conditions in the situations you describe. Here's a video of how me testing a 40 ft RV trailer that's had its chassis electrified with a variable AC bias from 40 to 120 volts: Hot Skin RV proximity test full scale - YouTube This simple NCVT check on microphones and backline stage amps will find anything biased with 40 volts or more AC above the earth plane.

If your genny isn't properly G-N bonded/earthed via a ground rod, and a hot-to-chassis short in a piece of stage gear electrifies your stage ground plane, this $20 tester will scream at you when it gets within a foot of anything large connected to a grounded power plug (like a mixing console or guitar amp) and within a few inches of any microphone or guitar on stage. And it will certainly scream at you if you get it within a foot of the genny itself that isn't properly earthed via a ground rod and "floating" above the earth plane. Proper genny grounding and bonding is very important to the safety of both your crew and gear as well as the performers themselves.

See No Shock Zone for a bunch of articles I've written about basic electrical principals. Some of these are quite basic, but for those who need a review of what voltage and amperage and wattage really means, this is a good primer. I also have bunch of YouTube videos at howtoseminars - YouTube that shows how to use test gear such as DMM's, clamp meters, why extension cords overheat and other cool stuff.

FYI: I have all of the major Ground Impedance Testers on my bench - Amprobe INSP-3, Ideal SureTest, and Extech CT-70 along with an "antique" Woodbridge GLIT I bought back in the late 70's with I first got my Master Electrician License, and plan on a comparison article pretty soon. I'll keep you posted.

 

[Mike Sokol]

Re: School is in

At my last tie-in, while I was metering, the venue electrician was trying to "help me" by switching my DMM to DC Volts.

Switching to Ohms works even better!!! :lol:

 

[Cody Hazelwood]

Re: The Test

The voltage divider rule is definitely the way to go. It made the math very easy!

 

[Tim McCulloch]

Re: School is in

Switching to Ohms works even better!!! :lol:

I was on a big industrial where the video tech was trouble-shooting a projector issue and he suspected the very long Edison cord that ran up the pick to the unit. He was up in a lift and asked to borrow a meter. As my Fluke case has a nice little strap to clip to the carabiner on his rope, I sent it up to him. He came down as said, "man, there is something really odd, I'm seeing wacky voltages at the end of that cable." I looked down at my meter and it was still on the power-up default, DC measurement scale. That it was finding DC (from a wall outlet) should have been disconcerting, too, but I digress.

When I pointed that out, he went back up and found correct voltage. He spent another hour working on the projector issue; I never found out what the final answer was.

In retrospect, I wish we could have taken some time to figure out if there really was DC present, or that the meter was simply trying to make sense of the input that it was given.

 

[Langston Holland]

Re: The Test

But... (using a water analogy) I wanted folks to see what was happening to the flow of electricity (current) as well as the pressure of electricity (voltage).

 

[Brian jojade]

Re: School is in

In retrospect, I wish we could have taken some time to figure out if there really was DC present, or that the meter was simply trying to make sense of the input that it was given.

If you have a volt meter set to DC and apply AC to it, it may show some random readings, depending on how it's sampling the voltage. Where it's sampling during the sine wave of the AC it may appear as DC to the meter for the duration of the sample. Problem is, the next sample could be anywhere else in the sine wave, causing the number to bounce all over the place.

On a typical analog old school meter, on the DC setting with an AC source, you would see the meter simply vibrate near zero volts.

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

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.

Here's the REALLY scary thing. Even if you had corner and tongue jack stands sitting on the dirt under your genny or production trailer, they offer such a high impedance path to earth that it's unlikely they'll have any grounding effect at all. A standard 8 ft ground rod buried in the earth can have an impedance as high as 25 ohms and still be within code. A great ground rod impedance for electronic assembly plants is maybe 5 ohms, and requires wet soil with salts added for conductivity on a regular basis. So 120 volts connected to a ground rod without benefit of a G-N bond will only draw perhaps 5 or 6 amps of current, (120 volts / 20 ohms = 6 amps), not enough to trip a 20 amp circuit breaker, but certainly enough to trip a GFCI if it was in the circuit.

I've not measured it in a while, but a few square inches of jack stand on the dirt will be in the hundreds (damp) or thousands (dry dirt) of ohms impedance. So a hot-to-chassis fault in a branch circuit somewhere will only send a few tens to hundreds of mA through the legs of the jack stands sitting on the dirt, which you'll never know about until you touch the generator panel while standing on wet grass. 8O~8-O~:shock:

And as you should be aware, the fault currents in a hot-to-chassis short should never go down the ground rod into the earth at all. These currents cycle back through the G-N bonding point in the service entrance, or at your generator panel, which is what actually trips a circuit breaker when you short a wire to the chassis. The earth itself has nothing to do with that shorting current. So without a proper Ground to Neutral bond with an earth connection from a ground rod, your stage’s “ground plane” can float around to whatever it likes, sometimes reaching 120 volts referenced to earth.

Of course, we humans are basically bags of salt water with a hand-to-hand impedance of around 1,000 ohms. You'll draw around 100 mA of current through your body if you stick one wet hand on a 120 volt electrified mic and the other on grounded guitar strings (or the other way around) which doesn't sound like a lot. But with just 10 mA alternating current through your arms it becomes difficult to let go of an electrified object because you're paralyzed by the current, and your hand grasping reflex is stronger than the opening muscles in your hand. That's why you get "stuck" when you've grabbed an electrified mic or instrument. And just 30 Volts AC which causes around 30 mA of current through your chest/heart is enough to put your heart into fibrillation. Without immediate CPR and an EAD (External Automated Defibrillator) you'll be brain dead in 5 minutes or so.

You should NEVER feel a shock on stage or standing at a mixer or lighting console. If you do, there's something seriously wrong with the grounding/bond somewhere in the power distro system, and it should be corrected immediately before somebody gets killed.

 

[Rob Timmerman]

Re: Why An Open Neutral Kills 120V Devices

A take home lesson:

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.

Yep. I like to use a large-ish test load (such as a 1kW PAR64) applied to one or more hot legs to verify that (a) voltage drop is within reason, (b) that I'm not getting any significant voltage rise on other phases when the load is applied, and (c) that the N-G voltage increases slightly (assuming I'm not measuring at the bonding point). Note that you can't determine (b) with a circuit analyzer that only looks at one phase.

 

[Mike Sokol]

Wild Leg

​

Yep. I like to use a large-ish test load (such as a 1kW PAR64) applied to one or more hot legs to verify that (a) voltage drop is within reason, (b) that I'm not getting any significant voltage rise on other phases when the load is applied, and (c) that the N-G voltage increases slightly (assuming I'm not measuring at the bonding point). Note that you can't determine (b) with a circuit analyzer that only looks at one phase.

Back in my mis-spent youth I built a 48 KW lighting system using (48) 1,000 watt PAR cans on 24, 2KW dimmers for my rock band. Since I had my Master Electrician license we would tie it into our own cam-locks I installed in the clubs service panel after measuring everything very closely. And I found that by turning on all the PAR's on one leg (24KW of load) we could get the neutral leg to drag over to the load side (as per Langston's great video) by up to 10 volts. I determined that the service panel had an undersized neutral wire, most likely because it was a converted flower mill which used 3-phase power for the heavy motor loads, and 3-phase motors don't produce any unbalanced neutral currents at all. So I could watch the heavy loaded leg of our lighting system drop to 110 volts while the light loaded leg rose to 130 volts. This is just the baby version of what happens when you open the neutral entirely.

The other part of this story is that the panel was wired as 3-phase Delta High-Leg, instead of 3-phase WYE. See High-leg delta - Wikipedia, the free encyclopedia for an overview. This was a wiring shortcut often done in the 50's and 60's for industrial buildings that needed mostly 240V 3-phase for motors, and some 120/240 volt single phase power for the office. Notice that if you meter from the Neutral center-tap to two of the legs you'll measure 120 volts, but from the neutral to the "Wild Leg" you'll measure 208 volts. They often call this the "Wild", "Red", or "High" leg for obvious reasons, and it's supposed to have orange tape around the feed wire as a warning. Now I was lucky enough to be working for Corning Glass at the time as my day job, so I knew exactly what a High-Leg 3-phase circuit was all about and I carefully metered the panel before installing cam locks. But one of the other bands who played there decided to take a pair of battery jumper cables (like you start your car with) and grab 120-volt power for their own lighting system because they were tired of tripping breakers. Unfortunately for them, they knew nothing about 3-phase power or Wild Leg transformers, so they tied their lighting system into the 208 volt "Wild Leg" and blew out a few dozen PARs in a second or two. I later asked their road guy why he didn't meter the panel first (let alone what he was doing inside of a live box), but found he didn't own a voltmeter of any kind. Of course this was the 70's, so some flames and sparks only added to the show (just kidding). :lol:

I had this same High-Leg Delta scenario happen at a campground in Texas last year. The camp "electrician" wanted to run some new 30-amp 120-volt power pedestals for additional campsites, and found one of the legs on the campground service panel was empty of circuit breakers. Thinking it was his lucky day, he plugged in a dozen 30 amp breakers to the empty (Wild Leg) buss and sent 208 volts to the 120 volt outlets at the campsites. A few new campers had pulled in with their RV's and were blowing up their RV appliances, but the campground electrician attributed it to a "lightning strike". It wasn't until a few days (and a few more trashed RV's) that somebody put a meter on an outlet and found 208 volts where there should be 120 volts. Yikes!!!!

The lesson is meter everything you can, every which way you can, and be suspicious of everything. I know I'm paranoid, but am I paranoid enough? :roll:

 

[TJ Cornish]

Re: School is in

Switching to Ohms works even better!!! :lol:

This is one of the many reasons I only use CATIII or better Flukes, which will not only keep me safe if I touch something I'm not supposed to, but they survive line voltage in Ohms mode.

 

[Mike Sokol]

Re: School is in

This is one of the many reasons I only use CATIII or better Flukes, which will not only keep me safe if I touch something I'm not supposed to, but they survive line voltage in Ohms mode.

Agreed.... I love my Fluke meters.

As you note, a high quality meter (such as a Fluke) has less of a chance of turning into a fireball of burning copper and melting your face off (called an arc flash). Poking meter leads around high amperage circuits is really dangerous since the energies involved are enough to vaporize meter components, test leads or a screwdriver in the wrong place. You've got to be careful around this high current stuff because the short circuit amperage can be far more dangerous than a 120 volt shock.

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

Yep. I like to use a large-ish test load (such as a 1kW PAR64) applied to one or more hot legs to verify that (a) voltage drop is within reason, (b) that I'm not getting any significant voltage rise on other phases when the load is applied, and (c) that the N-G voltage increases slightly (assuming I'm not measuring at the bonding point). Note that you can't determine (b) with a circuit analyzer that only looks at one phase.

For anybody not familiar with the concept of voltage drop on long runs, here's a pretty good primer I wrote for RV owners who run appliances on long extension cords: RV Electrical Safety: Part VI – Voltage Drop | No~Shock~Zone

And here's a handy chart from SurgeX with worldwide voltages and frequencies: SurgeX International - The World's Most Trusted Name in Power Protection

 

[gordon mcgregor]

Re: Why An Open Neutral Kills 120V Devices

Those little Fluke beepers have saved many a domestic appliance engineer from a bad day me included, one thing do you guys bond stages to the ground rod of a genny or if it's too far away connect the stage to it's own ground rod? G

Ps Mike if your testing those earth impedence testers check how they react if here is voltage on the earth, a common unit here shows the same indiction if there is voltage on the earth as it does for no earth connection, fine if you then meter it but not so good if you don't

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

Those little Fluke beepers have saved many a domestic appliance engineer from a bad day me included, one thing do you guys bond stages to the ground rod of a genny or if it's too far away connect the stage to it's own ground rod? G

I'm actually writing a tutorial about festival generator power, and bonding the stage to the same generator ground rod is on my hit list as a recommendation. But that's an entire article I'm currently doing research for. Much more on that subject later....

Ps Mike if your testing those earth impedence testers check how they react if here is voltage on the earth, a common unit here shows the same indiction if there is voltage on the earth as it does for no earth connection, fine if you then meter it but not so good if you don't

My B&K 309 Digital Earth Resistance Tester (say that 3 times fast...) uses a 400 Hz test tone and high-pass filter, so it ignores any 60 Hz currents already flowing through the ground rod from your AC power. So according to their directions, you don't need to disconnect your ground rod or power down your building for a ground rod impedance test.

Model 309, Digital Earth Resistance Meter - B&K Precision

 

[gordon mcgregor]

Re: Why An Open Neutral Kills 120V Devices

The reason I asked this was because I have seen these things getting used to test for safe isolation as well as ELI tests and of course when the power gets switched off the lights go out on the tester but there could still be voltage on the earth. Here's alink to the device in question Socket and See & See PDL234 Plus Part P Loop Testing Kit
these are widely used in the UK as you have to test the Earth Loop Impedence when you work on any electrical circuit, however some people use them to test for safe isolation and mis-interperate the earth fault light as meaning ONLY no earth with potentionaly bad results. G

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

The reason I asked this was because I have seen these things getting used to test for safe isolation as well as ELI tests and of course when the power gets switched off the lights go out on the tester but there could still be voltage on the earth. Here's alink to the device in question Socket and See & See PDL234 Plus Part P Loop Testing Kit
these are widely used in the UK as you have to test the Earth Loop Impedence when you work on any electrical circuit, however some people use them to test for safe isolation and mis-interperate the earth fault light as meaning ONLY no earth with potentionaly bad results. G

Gordon,

This will be an interesting thing for me to study. I've done a lot of sound work with MHA Audio here in the states, some with a totally British Tannoy sound system originally used for AC-DC world tours (The band, not the current). So there was a lot of UK hookups and such on the FOH and monitor gear. I do know that Canadian grounding is completely different from USA grounding, but I've not thought too much about UK grounding until now. It will be fun to get my head around it a bit.

 

[brian maddox]

Re: Why An Open Neutral Kills 120V Devices

Gordon,

This will be an interesting thing for me to study. I've done a lot of sound work with MHA Audio here in the states, some with a totally British Tannoy sound system originally used for AC-DC world tours (The band, not the current). So there was a lot of UK hookups and such on the FOH and monitor gear. I do know that Canadian grounding is completely different from USA grounding, but I've not thought too much about UK grounding until now. It will be fun to get my head around it a bit.

It's funny... I've been reading all this stuff and thinking about mike scarfe's rig and its unusual powering and somehow never put it together that you're from hagerstown and would have to have worked with that rig before...

Anyway, now that I know you're basically up the street I know where to go if i need some well versed power help...

 

[Tim McCulloch]

Re: Why An Open Neutral Kills 120V Devices

Hagersbush, MD? ( LOL )

Is he still running the Martin rig?

 

[brian maddox]

Re: Why An Open Neutral Kills 120V Devices

Hagersbush, MD? ( LOL )

Is he still running the Martin rig?

As a card carrying citizen of beautiful FredNeck MD, I resemble that comment...

Far as I know mike is still Doc Martin.... But I still love his old weird AC/DC rig with all the 8inch drivers. Still a really good sounding box. And he still has an XL4. Triple yay!

 

[Mike Sokol]

Re: Why An Open Neutral Kills 120V Devices

As a card carrying citizen of beautiful FredNeck MD, I resemble that comment...

Far as I know mike is still Doc Martin.... But I still love his old weird AC/DC rig with all the 8inch drivers. Still a really good sounding box. And he still has an XL4. Triple yay!

Yeah, Scarfe is mostly Martin now. But his original AC/DC M6 rig with all those Tannoy cones staring at you was a pretty awesome site to behold. The more cabinets you stacked up, the more the bass coupled together so there were no subwoofers required even though it was all 10" (correction, not 8") drivers. And the mix of 240 volt amp racks and 120 volt drive racks was interesting, to say the least. But we could use that system for anything from a symphony to a hip-hop rave, and it always got the job done. Did I mention it was LOUD!