Diary of a generator operator— Part 3
Or how my IA training kept me one step ahead of disaster.
Figure 1 – Since the neutral is bonded to the equipment grounding conductor at the generator, we can measure the combined impedance of the ground and neutral conductors at the test jacks at the last distro box.
IN THIS INSTALLMENT of “Diary of a Generator Operator,” I pick up the story of how the analytical tools I learned in IATSE Local 481’s Training, Education, and Classification (TEC) program, enabled me to stay one step ahead of a looming disaster on the set of a major motion picture. From what I learned, I was able to isolate the problem to a bad piece of 4/0 cable in our neutral run, but not until several 18ks failed, and a 1400 A generator blew up. The bad piece of 4/0 created a resistive neutral that turned our three-phase distro into a voltage divider circuit where the voltage of each phase floated in an inverse relation to its load. As demonstrated in Parts 1 and 2, a bad piece of 4/0 cable has the potential to bring even the largest production to a crashing halt. In this part, I explore what preventive steps can be taken to identify a resistive neutral at the outset and avoid these hazards. Now, back to my diary.
Post Script: I got my last paycheck from the show in the mail, which got me thinking again about what happened on set. I still find it hard to believe that a single piece of 4/0 cable could create such havoc.
It’s hard to believe it was the cause of our 18k ballasts not striking; the cause of our voltage floating; the reason our generator crapped out. There must be preventive steps that can be taken to avoid these hazards? Of course! A Rigging Gaffer can identify a resistive neutral, before even energizing the set, by measuring the resistance between neutral and ground at the last distro box at the end of the line.
Since the neutral and ground are bonded at the generator, they form a continuous circuit. The test probes of a multimeter set to read resistance will create a potential between the ground and neutral of the box. The resistance of the wire between these two points (the ground wire back to the generator, the neutral/ground bonding jumper, and the neutral wire from the generator back to the last distro box), limits the current to a particular value. With the potential and current values, the meter can calculate the resistance of this circuit using Ohm’s Law (V=IR, or R=V/I). Given that the resistance of a 100' stick of 4/0 is generally about 0.005Ω, a reading of more than 2 – 3 Ω is an indication of a pinch point somewhere in the circuit that will cause trouble when the system is energized. But, this method has several obvious drawbacks to it. What this technique does not tell you is whether that pinch point is in the ground or in the neutral. Either is a problem from a safety standpoint and warrants closer inspection.
Why a high resistance in the neutral is problematic is evident from my experience described in Parts 1 and 2 of this series. Why high resistance in the ground is a problem is less evident, but becomes clear when you consider how the equipment grounding conductor (EGC) of a distro system works. As stipulated by NEC Article 110-10: “Circuit Impedance and Other Characteristics,” the purpose of the EGC is to create sufficient fault current to trip the over current protective device and remove the source of voltage before it can cause damage. NEC Article 110-10: “Circuit Impedance and Other Characteristics” reads as follows: “The over-current protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit.”
To create sufficient fault current, called the available short circuit current (ASCC), the EGC must have a low impedance. According to Ohm’s Law (V=IR), the lower the impedance, the higher the fault current generated, the faster the breaker trips. Unfortunately, the inverse is also true: If a piece of ground cable has a high resistance, the ASCC it generates may not be sufficient to trip the breaker, leading to the possibility of sustained fault current that can cause serious damage, or worse, administer a lethal shock. This becomes clear when you draw it out as a circuit diagram (Figure 2).
The impedance of a ground fault circuit consists of the resistance of the distribution cable between the generator and the point of the ground fault (R 1 ), plus the resistance of the short to ground (RSC), plus the resistance of the EGC (R E ) back to the ground/neutral bond in the generator. According to Ohm’s Law (V = IR), the ASCC generated is in inverse proportion to the impedance of the ground fault circuit, i.e. an increase in the impedance of the EGC results in less fault current. This becomes clear when we restate Ohm’s Law to find the fault current. In this case, I = V / (R 1 + R SC + R E ). In this form we see that if R SC or R E are high the circuit breaker in our diagram will not open. If R SC is low and R E high a fire will likely start.
To ensure that breakers will trip in a timely fashion in the event of a ground fault, ground impedance should be less than 1Ω (preferably in the 0.25 Ω region). Since the impedance of a 100' stick of 4/0 feeder is typically about .005Ω, precision low-value resistance measurements become necessary when testing cables intended to carry significant current, or, as in our case, when extremely high reliability must be ensured. Unfortunately, it is not easy to accurately measure the impedance of a system ground, or its component parts individually.
To accurately measure such low impedance values requires the use of the Kelvin 4-Wire method of resistance measurement in which a current is injected in the circuit and the voltage drop across the load is measured and used to calculate the resistance. Otherwise, you are simply measuring the resistance of the meter’s test leads which amount to about 0.2Ω(see sidebar for details). Unfortunately, a 4-wire system is not very practical when it comes to measuring the resistance of the EGC of an entire distro system. That makes the 2-wire method of measuring impedance, even with a 0.2Ωerror introduced by its leads, a more practical approach by default. Besides, by the time the impedance of the ground system becomes an issue (> 2 Ω), the 0.2Ωerror introduced by the meter’s test leads is negligible and can be disregarded. A final drawback to this method is that the distro system cannot be energized (see Figure 3 for details), which means taking measurements will delay energizing the set. Not an option.
Impedance (Z) measurements
A circuit analyzer’s ability to measure impedances of a hot distro make it a particularly valuable tool when it comes to identifying the cause of a resistive neutral. If high neutral impedance is measured at the further most box, a circuit analyzer can help you to isolate the source of the impedance. To determine where the problem is, measure neutral impedance at the nearest open pockets to the generator (whether it’s a spider box, open pocket of a three-fer, or distro box). If high neutral impedance is not measured, work your way out incrementally, testing the next open pockets until high impedance is measured. You now know that the bad piece of cable or distro equipment is between the point where you measured high impedance and the last point where you did not. Replacing the distro between these two points should eliminate the problem. (Sometimes high resistance points can be identified using an infrared (IR) thermometer to detect hot spots, or a voltmeter to detect an excessive voltage drop across a piece of distro equipment.)
Available short circuit current (ASCC) measurements:
To check that there is sufficient available short circuit current (ASCC) to trip breakers in the event of a ground fault, a circuit analyzer can also measure the impedance of the grounding conductor and calculate the ASCC. The meter calculates the ASCC by dividing the line voltage by the circuit’s total impedance [ASCC = Line Voltage / (Hot Impedance + Neutral Impedance + Ground Impedance)]. To assure the quick elimination of shorts, the measured ASCC should be more than 10x the rating of the nearest upstream breaker. (Note: To measure ground impedance, the meter shunts a small amount of current to the ground conductor, which will trip a GFCI. Which makes a circuit analyzer a great means of also testing the operability of GFCIs to assure that they will work in the event of a ground fault.)
Voltage drop (VD) measurements:
Circuit analyzers also measure voltage drop. To determine voltage drop, a circuit analyzer measures the line voltage with no load, measures the voltage under load, and then calculates the voltage drop between the loaded and unloaded circuit.
An efficient branch circuit should have less than 5% voltage drop at the furthest receptacle from the power source. When voltage drop from the supply to the load is too high (greater than 5%), a circuit analyzer will enable you to locate the problem. First, measure the impedance of the hot and neutral. If one impedance measurement is exceedingly higher than the other then the problem is with the conductor that shows the higher impedance (using the method described above will enable you to pinpoint the exact source of the high impedance).
If a voltage drop measurement exceeds 5% but noticeably decreases as the testing moves closer to the power source, then the circuit may have an undersized wire, too long a cable run, or excessive current on the circuit. Check the wires to ensure that they are sized per Code and measure the current on the branch circuit. If a voltage drop reading changes significantly from one receptacle to the next, then the problem is a high impedance point at or between two of the receptacles. It is usually located at a termination point, such as a bad splice or loose wire connection, but could also be a faulty piece of feeder cable.
While the measurement loads of circuit analyzers (12A, 15A, and 20A loads at 120V) are not sufficient to determine the voltage drop on a fully loaded distro system, it can however indicate the potential for severe voltage drop under larger loads. A circuit analyzer’s remaining function, measuring line frequency, is also important information for a generator operator. All together a circuit analyzer is a versatile tool and has a place in every generator operator’s toolkit.
This is the final installment of “Diary of a Generator Operator.”
Guy Holt has served as a gaffer, set electrician, and generator operator on numerous features and television productions. He is recognized for his writing on the use of portable generators in motion picture production (available soon in book form from the APT Press). Guy has developed curriculums on power quality and electrical hazard protection that he has taught through the IATSE Local 481 Electrical Department’s “TECs”
Program. He is the owner of ScreenLight & Grip, a motion picture lighting rental and sales company that specializes in innovative approaches to set power using Honda portable generators.
For an unabridged version of Part 3, visit http://screenlightandgrip.com/html/hd_plug-n-play_pkg.html.
Post Script: I got my last paycheck from the show in the mail, which got me thinking again about what happened on set. I still find it hard to believe that a single piece of 4/0 cable could create such havoc.
It’s hard to believe it was the cause of our 18k ballasts not striking; the cause of our voltage floating; the reason our generator crapped out. There must be preventive steps that can be taken to avoid these hazards? Of course! A Rigging Gaffer can identify a resistive neutral, before even energizing the set, by measuring the resistance between neutral and ground at the last distro box at the end of the line.
Since the neutral and ground are bonded at the generator, they form a continuous circuit. The test probes of a multimeter set to read resistance will create a potential between the ground and neutral of the box. The resistance of the wire between these two points (the ground wire back to the generator, the neutral/ground bonding jumper, and the neutral wire from the generator back to the last distro box), limits the current to a particular value. With the potential and current values, the meter can calculate the resistance of this circuit using Ohm’s Law (V=IR, or R=V/I). Given that the resistance of a 100' stick of 4/0 is generally about 0.005Ω, a reading of more than 2 – 3 Ω is an indication of a pinch point somewhere in the circuit that will cause trouble when the system is energized. But, this method has several obvious drawbacks to it. What this technique does not tell you is whether that pinch point is in the ground or in the neutral. Either is a problem from a safety standpoint and warrants closer inspection.
Why a high resistance in the neutral is problematic is evident from my experience described in Parts 1 and 2 of this series. Why high resistance in the ground is a problem is less evident, but becomes clear when you consider how the equipment grounding conductor (EGC) of a distro system works. As stipulated by NEC Article 110-10: “Circuit Impedance and Other Characteristics,” the purpose of the EGC is to create sufficient fault current to trip the over current protective device and remove the source of voltage before it can cause damage. NEC Article 110-10: “Circuit Impedance and Other Characteristics” reads as follows: “The over-current protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit.”
To create sufficient fault current, called the available short circuit current (ASCC), the EGC must have a low impedance. According to Ohm’s Law (V=IR), the lower the impedance, the higher the fault current generated, the faster the breaker trips. Unfortunately, the inverse is also true: If a piece of ground cable has a high resistance, the ASCC it generates may not be sufficient to trip the breaker, leading to the possibility of sustained fault current that can cause serious damage, or worse, administer a lethal shock. This becomes clear when you draw it out as a circuit diagram (Figure 2).
The impedance of a ground fault circuit consists of the resistance of the distribution cable between the generator and the point of the ground fault (R 1 ), plus the resistance of the short to ground (RSC), plus the resistance of the EGC (R E ) back to the ground/neutral bond in the generator. According to Ohm’s Law (V = IR), the ASCC generated is in inverse proportion to the impedance of the ground fault circuit, i.e. an increase in the impedance of the EGC results in less fault current. This becomes clear when we restate Ohm’s Law to find the fault current. In this case, I = V / (R 1 + R SC + R E ). In this form we see that if R SC or R E are high the circuit breaker in our diagram will not open. If R SC is low and R E high a fire will likely start.
To ensure that breakers will trip in a timely fashion in the event of a ground fault, ground impedance should be less than 1Ω (preferably in the 0.25 Ω region). Since the impedance of a 100' stick of 4/0 feeder is typically about .005Ω, precision low-value resistance measurements become necessary when testing cables intended to carry significant current, or, as in our case, when extremely high reliability must be ensured. Unfortunately, it is not easy to accurately measure the impedance of a system ground, or its component parts individually.
To accurately measure such low impedance values requires the use of the Kelvin 4-Wire method of resistance measurement in which a current is injected in the circuit and the voltage drop across the load is measured and used to calculate the resistance. Otherwise, you are simply measuring the resistance of the meter’s test leads which amount to about 0.2Ω(see sidebar for details). Unfortunately, a 4-wire system is not very practical when it comes to measuring the resistance of the EGC of an entire distro system. That makes the 2-wire method of measuring impedance, even with a 0.2Ωerror introduced by its leads, a more practical approach by default. Besides, by the time the impedance of the ground system becomes an issue (> 2 Ω), the 0.2Ωerror introduced by the meter’s test leads is negligible and can be disregarded. A final drawback to this method is that the distro system cannot be energized (see Figure 3 for details), which means taking measurements will delay energizing the set. Not an option.
Impedance (Z) measurements
A circuit analyzer’s ability to measure impedances of a hot distro make it a particularly valuable tool when it comes to identifying the cause of a resistive neutral. If high neutral impedance is measured at the further most box, a circuit analyzer can help you to isolate the source of the impedance. To determine where the problem is, measure neutral impedance at the nearest open pockets to the generator (whether it’s a spider box, open pocket of a three-fer, or distro box). If high neutral impedance is not measured, work your way out incrementally, testing the next open pockets until high impedance is measured. You now know that the bad piece of cable or distro equipment is between the point where you measured high impedance and the last point where you did not. Replacing the distro between these two points should eliminate the problem. (Sometimes high resistance points can be identified using an infrared (IR) thermometer to detect hot spots, or a voltmeter to detect an excessive voltage drop across a piece of distro equipment.)
Available short circuit current (ASCC) measurements:
To check that there is sufficient available short circuit current (ASCC) to trip breakers in the event of a ground fault, a circuit analyzer can also measure the impedance of the grounding conductor and calculate the ASCC. The meter calculates the ASCC by dividing the line voltage by the circuit’s total impedance [ASCC = Line Voltage / (Hot Impedance + Neutral Impedance + Ground Impedance)]. To assure the quick elimination of shorts, the measured ASCC should be more than 10x the rating of the nearest upstream breaker. (Note: To measure ground impedance, the meter shunts a small amount of current to the ground conductor, which will trip a GFCI. Which makes a circuit analyzer a great means of also testing the operability of GFCIs to assure that they will work in the event of a ground fault.)
Voltage drop (VD) measurements:
Circuit analyzers also measure voltage drop. To determine voltage drop, a circuit analyzer measures the line voltage with no load, measures the voltage under load, and then calculates the voltage drop between the loaded and unloaded circuit.
An efficient branch circuit should have less than 5% voltage drop at the furthest receptacle from the power source. When voltage drop from the supply to the load is too high (greater than 5%), a circuit analyzer will enable you to locate the problem. First, measure the impedance of the hot and neutral. If one impedance measurement is exceedingly higher than the other then the problem is with the conductor that shows the higher impedance (using the method described above will enable you to pinpoint the exact source of the high impedance).
If a voltage drop measurement exceeds 5% but noticeably decreases as the testing moves closer to the power source, then the circuit may have an undersized wire, too long a cable run, or excessive current on the circuit. Check the wires to ensure that they are sized per Code and measure the current on the branch circuit. If a voltage drop reading changes significantly from one receptacle to the next, then the problem is a high impedance point at or between two of the receptacles. It is usually located at a termination point, such as a bad splice or loose wire connection, but could also be a faulty piece of feeder cable.
While the measurement loads of circuit analyzers (12A, 15A, and 20A loads at 120V) are not sufficient to determine the voltage drop on a fully loaded distro system, it can however indicate the potential for severe voltage drop under larger loads. A circuit analyzer’s remaining function, measuring line frequency, is also important information for a generator operator. All together a circuit analyzer is a versatile tool and has a place in every generator operator’s toolkit.
This is the final installment of “Diary of a Generator Operator.”
Guy Holt has served as a gaffer, set electrician, and generator operator on numerous features and television productions. He is recognized for his writing on the use of portable generators in motion picture production (available soon in book form from the APT Press). Guy has developed curriculums on power quality and electrical hazard protection that he has taught through the IATSE Local 481 Electrical Department’s “TECs”
Program. He is the owner of ScreenLight & Grip, a motion picture lighting rental and sales company that specializes in innovative approaches to set power using Honda portable generators.
For an unabridged version of Part 3, visit http://screenlightandgrip.com/html/hd_plug-n-play_pkg.html.