Most properties today, whether residential, commercial, or industrial, include at least one building or structure on that property. Often there are multiple buildings on a single property. Some include buildings that are each supplied by its own utility service and others have an electrical service at one point and deliver electrical power to the other buildings or structures by feeder(s) or by branch circuit(s). This article takes a closer look at the grounding and bonding requirements and methods for separate buildings or structures supplied from other than a service. //

The second method for grounding and bonding at a separate building or structure is allowed where the feeder does not provide an equipment grounding conductor, but does include a system grounded (often a neutral) conductor. This second method is a bit more difficult to utilize because there are more specific restrictions that must be considered and adhered to. Three conditions must exist before one may use the grounded conductor for grounding purposes at a separate building or structure.

Figure 4. Grounding and bonding at separate buildings or structures using the grounded conductor by the method specified in Section 250.32(B)(2)

The first condition is that an equipment grounding conductor, of any form specified in 250.118, is not provided or run with the supply to the structure. This means that only the phase conductor(s) and the system grounded conductor either as direct burial, in nonmetallic conduit underground, or as overhead conductors are included. The key is that no equipment grounding conductor is included.

The second condition is that no continuous metallic paths exist or are otherwise present, and that are bonded to the grounding system in both buildings. Examples of continuous metallic paths could be metal water piping, building steel, metallic conduit, cable shields, metal ducts, and so forth.

The last condition that must be met prior to utilizing this method is that no equipment ground-fault protection is installed on the supply service or feeder, as neutral-to-ground connections on the load side of this equipment can nullify or desensitize the equipment protection.

If all of these conditions are met, then the grounded conductor of the feeder or branch circuit is permitted to be used for grounding and bonding the electrical equipment. It must be connected to the structure disconnecting means enclosure to which the required grounding electrode conductor is also connected. The minimum size of the grounded conductor of the feeder or branch– circuit must satisfy two minimum sizing requirements. First, it must be adequate to carry the maximum load on the grounded (often a neutral) conductor as specified in 220.22. Second, it also must not be smaller than the required equipment grounding conductor for the feeder or branch circuit using 250.122, based on the size of fuse or circuit breaker ahead of it.

We all know that water in an electrical system is bad news. And we do our best to keep it out by specifying waterproof cable and connectors, and following industry best practices for installation and maintenance.

So, what if water does get into a coaxial radio frequency (RF) network? Unfortunately, its presence is not always obvious and its impact can be elusive and difficult to manage. Here are some tips to help you trouble-shoot a persistent moisture problem: //

The presence of direct current (DC) voltage on the line also speeds up the corrosion process if a steady supply of moisture is available. A copper-clad steel center conductor can completely dissolve in less than a day when submerged in water while voltage is applied.

jcdpk
22 posts · Joined 2011

#65 · Sep 10, 2013

Commonly used DC test voltages for AC equipment are:
AC equipment rating DC test voltage
Up to 100 VAC 100 and 250 VDC
440 to 550 VAC 500 and 1000 VDC
2400 VAC 1000 to 2500 VDC
4160 VAC and above 1000 to 5000 VDC (or higher)

So, what is “good” insulation? Since we know that insulation has a high resistance to current flow, “good” insulation must be able to provide a high resistance to current flow and be able to maintain that high resistance over a long period of time. In order to evaluate the quality of the insulation certain standard tests have been developed which provide a reliable indicator to determine what comprises “good” insulation.

There are two tests that the production technician can easily perform using a battery powered megger like the one shown in Figure 29. The first test is the short time or spot reading test and the second test is the one minute test.

The spot reading test
In the spot reading test you simply connect the “earth” lead of the megger to a good ground and the “line” lead to the conductor and operate the megger for a short time, say for 30 seconds or so. If the apparatus you are testing has a very small capacitance, such as a short run of cable, the spot reading is all that is necessary. However most equipment (like electric motors and long runs of electrical cable) is capacitive, so the very first spot reading can be only a rough guide as to how good or how bad the insulation is.
Bear in mind that the temperature and humidity will affect the readings and electrical circuits do not have to read infinity (perfect insulation on the megger scale) for the circuit to be serviceable. The NEMA standard for minimal insulation resistance is: 1 megohm per rated KV plus 1 megohm. What that means is that if you have a 1000 volt circuit, you should have a minimum of 2 megohms to ground for the circuit to be considered safe to energize and operate. If you have a 4000 volt circuit, you need (5 megohms) and if you have a 460 volt circuit, you should have (1-1/2 megohms), and so on. Insulation that is in good condition will normally have 40 - 50 megohms, or more, to ground.

The one minute reading test
The other test is the one minute reading. This method is fairly independent of the influence of temperature. It is based on the current absorption of good insulation compared to the current absorption of moist or contaminated insulation. A characteristic of good insulation is that it will show a continual increase in resistance (which means less leakage current is flowing) over a period of time. The initial test current (called the absorption current) is absorbed by the capacitance of the equipment being tested and then after that, any current flowing is the leakage through the insulation. If the insulation contains much moisture or other contaminants, the absorption current is masked by a high leakage current which stays at a fairly constant value, keeping the resistance low.
The value of the one minute test is that it can give you a better idea as to the condition of the insulation and alert you to a problem even when the spot reading indicates that everything is OK.
For example, let’s say a spot reading on a 460 volt induction motor was 10 megohms, which at first glance is well above the minimum requirement. Now lets assume that the one minute test showed the resistance quickly climbing to 10 megohms and then from there, holding steady for the rest of the 60 seconds. This means that there may be dirt or moisture on the windings. On the other hand, if the reading gradually increases between the 30 and 60 second time interval, then you can be reasonably certain that the windings are in good condition.
The comparison of the 30 second reading to the 60 second reading is called the dielectric absorption ratio (or D.A.R.). The way the ratio is calculated is to divide the 60 second reading by the 30 second reading. Using that method, the chart in Figure 30 will give you an idea of how to determine if the insulation is good.

Insulation

condition

60/30 second

ratio

Poor

Less than 1

Questionable

1.0 - 1.25

Good

1.4 – 1.6

Excellent

Above 1.6*​

*In some cases, with motors, values approximately 20% higher than shown here indicate a dry, brittle winding which will fail under shock conditions or during starts. For preventive maintenance, the motor winding should be cleaned, treated and dried to restore winding flexibility.

Dielectric absorption ratio chart
There is another megger test called the “Ten minute test” which is similar to the one minute test. Because this test requires ten minutes to perform, it is better accomplished by line operated (120 volt) equipment. Essentially the test is performed for ten minutes, the ten minute reading is divided by the one minute reading and the resulting ratio is called the Polarization Index. This test is mostly used on larger equipment that has large capacitance and requires longer time to stabilize the absorption test current. The conclusions drawn from the test are the same as those drawn from the dielectric absorption ratio but the actual ratio values of the polarization index are not the same as for the dielectric absorption chart in Figure 30.
Using the some of the symptoms listed in the “Problem” column of the troubleshooting chart, let’s go through the process that should be followed to perform the tests and develop the solution
Breaker trips free when motor start is attempted.

If the circuit breaker trips free (or trips during normal motor operation) it should not be re-closed and another motor start attempted before testing the motor and the power cables for an insulation failure. If there is a problem, additional starts will just cause further damage.
The symptom of the circuit breaker tripping is always caused by overcurrent; either a phase to phase short circuit or a phase to ground short circuit.

Insulation appears as a bunch of capacitors. Good insulation appears almost constant with voltage. Deteriorated or contaminated insulation megger readings drop as voltage increases. The standard measurement is the dielectric absorption ratio which is the 10 minute divided by the 1 minute reading. Typically a good number is 3-5. Megger tests need a constant (NOT hand cranked) voltage source and take readings after one minute to be valid. And you need to ground (short) for 3 times that time if you've already taken readings once. The DAR test gives hint why. Repeatedly meggering shows an increase no matter what the voltage (even at the same voltage) if the system isn't grounded for 3 times the test interval in between tests.

A lot of guys just stick the megger on there and take less than a 10 second reading. That's not a valid reading but hey all I'm looking for is over 5 megaohms org 100 megaohms as per the current IEEE standard temperature corrected after 1 minute but if in 10 seconds I've already got 20+, I don't need to continue the test. So I do it too but the closer I get to 5 or 100 megaohms the more I start watching the test time to get a by-the-book reading if the test is marginal. So if what I'm doing gets passed on the next guy might think A 10 second reading is valid when it's not...it's just close enough for spot check. As I megger longer the reading will climb, fast at first then slowly eventually. If there is moisture or contamination it will be very erratic. With very accurate readings you can see stair steps as the cracks charge up one at a time. Watching the readings or graphing them can be just as useful as the number.

And you can't "increment" it except for tip up (step bvoltage) tests where DAR is a better test anyways so step voltage tests are unnecessary except by customer request. And ever try measuring each of the three leads on a 6 or 9 lead motor nd wonder why the readings "go up"? I've seen that done too. It also happens when I megger an open frame breaker but again...I'm doing pass fail. Precise readings aren't needed. Hand cranked is OK for that but not for accuracy. Not only that but at one time and cranked megger were $500 and digital battery powered ones were $5000. Now you can get a decent Amprobe (a division of Fluke) or Extech (a division of Flir) for under $200 new and the AEMCs nd Biddle/Megger brands falling in the range of around $300-500 so there is no justification for hand cranked anymore.

As to what is valid, IEEE 43 gives 500 V for everything under 1000 V. Up to 12.5 kV, 1000 V is plenty. So you can just use one of the cheap common meggers to test everything until you get out of motors nd generators and out deep into distribution voltage territory. Up to 600 V, all insulation by ICEA/NEMA/UL is actually tested (megger) to some crazy value like 3 times the rating plus some more so even 150 V insulation (the lowest quality rating that has to be special ordered) is still rated higher than what a megger puts out so there is no reason for true test to use 100 or 250 V, again using a 1 minute reading. Use either a built in timer or your phone to time it.

The exception is VFDs. Following the manufacturer instructions you inside before meggering. But knowing it has blocking in the forward nd reverse voltage directions I sometimes drop to 250 V on a 460 V motor or 100 V on a 230 V motor just so that I can save time on a spot check and megger without unwiring. If the drive/motor fails (shorted) I know I have a bad motor/cable/drive anyway and step two is unwiring it all and running diode tests on the DC and AC busses to test the antiparallel diodes for shorts. So I am not only cheating but directly ignoring manufacturers instructions that tell you that you will destroy a drive by meggering it. There is a voltage that is really close to the AC peak voltage where particularly with SCRs self commutation occurs. Even without voltage on the gates, a high enough voltage on a power semiconductor will cause it to start conducting nd once it starts with some of them, they won't stop until they re damaged or power is removed. Damage with a battery powered megger is probably impossible but it leaves the possibility open and shows a smart customer you don't know what you are doing.

So yeah I'm not surprised at all. First unless you have a special case there is no reason for a 100 or 250 V test. Second if you didn't discharge fully...and I mean shorting it with a ground clamp for 5 minutes or more between tests, you're just seeing residual charges building up as your test gradually looks more like the 10 minute result. I'll bet if you went down in voltage or started over you would continue to see increases. The way a high end meter tests for this is that it measures residual voltage and grounds the leads until it goes away. And you get just ground a little...it takes much longer to drain a capacitor at a very low voltage compared to charging it at a very high voltage where the voltage difference helps speed the process up. //

• NFPA 99 requires special protection against electrical shock in facilities designated as “wet procedure locations.”
• Wet procedure designations are based upon risk assessments that consider types of procedures conducted, electrical equipment deployed and liquid-mitigation protocols.

During a routine arthroscopic shoulder repair, an operating table’s electrical panel short circuited, causing the table to suddenly decline. The surgical team swiftly moved the anaesthetized patient to a transport trolley before concluding the procedure, avoiding a potentially catastrophic outcome.

All this happened while intravenous fluid was leaking onto the floor. Despite mopping and suctioning, the surgical team stood over a wet film of water — a truly hazardous situation.

Electrical accidents within operating rooms are rare. Still, this incident — documented recently in the Journal of Anesthesiology Clinical Pharmacology — demonstrates why electrical precautions must remain ever-present concerns.