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. //

Active power, P

Active power is expressed in watt (W). Sometimes this power is also called “real power.” This is the power you are actually consuming.

Reactive power, Q

Reactive power is expressed in volt-ampere reactive (var)
This power is stored in components, then released again back to the source through the AC cycle. Capacitors and inductors do this.

Apparent power, S

Apparent power is expressed in volt-ampere (VA)
(RMS voltage times the RMS current). A power supply must be capable to output the full apparent power delivered to a circuit, not just the active power.

When looking at a typical phasor diagram on a meter, it is assumed the current phasors are rotating counter clockwise, with the voltage phasors stationary at their pre-determined locations. For demonstration purposes, Figure 1 is a 4 wire wye configured system, with ABC rotation.  Phase A voltage, or Va, lays on the primary “x” axis at 0° phase shift.  Ia is laying on top of the voltage phasor at 0° as well, indicating unity power factor, or a power factor of 1.  Phase B and C voltages and current pairs are separated by 120 degrees.

When a phase shift between the voltage and current occurs, this is due to reactance on the load, typically in the form of inductance.  This creates a lagging power factor.  When something is referenced as leading or lagging, this is a means of relating the phase relationship of the current waveform to the voltage waveform, and is always with reference to what the current is doing.  ///

quadrants

Wednesday, March 10, 2010
Power

Over the past three weeks ELWA has been hosting a group of people from the US that have been helping to make upgrades to the power system. The team consists of former missionaries, former missionary kids from ELWA, electricians, contractors and even a Liberian who used to work at ELWA but now lives in Philadelphia. In total 9 people traveled out from the US to work with ELWA's 5 electricians and an extra 8-10 ELWA services staff that helped out.

The project consisted of replacing all the transformers on campus so the voltage could be dropped from 7,200 volts to 2,400 on the primary transmission lines. The change was recommended a couple years ago after an electrical engineer analyzed ELWA's system and recommended the voltage be dropped to reduce the high voltage leaking to ground and jumping around insulators. We also installed some new transmission lines and changed our output voltage that the generators produce.

GENERAL

In the execution of its mandate to provide adequate and reliable electric power to the nation at economically reasonable tariff, the Liberia Electricity Corporation (LEC) operates and maintains two (2) distinct electrical power system, namely: the Monrovia Power system and the rural Electrification system. The Monrovia power system before the war supplied electricity to Monrovia and its outlying areas, extending to Kakata City , Tubmanburg City, and Buchanan City . Rural electrification before the war operated eleven (11) isolated diesel out stations with three under construction at the onset of the civil war, served the people who resided out side the Monrovia power system.

THE EVOLUTION OF LEC

In the early 1940s, the Monrovia Power system consisting of a single unit, serving the public. The unit was located at the corner of Carey & Lynch streets and was operated by Henry F. Luke, after whom the Luke Power plant at Bushrod Island is named. Monthly collection then never exceeded 16% of the monthly billing.

In the year 1949, the Government of Liberia (GOL) procured three 40-kW superior diesel generators through the United States Government Land Lease Program, and installed them at the Krutown power plant where the LEC central office is located today.

The Liberian company led by Commander William R. Trimble under contract with the GOL, replaced the Liberia Company and operated the Krutown power plant until 1960.

In June 1960, the Monrovia Power Authority (PUA) was created by law to consolidate and control the activities associated with power generation, transmission and distribution with the view to reducing system technical and commercial losses. The Stanley Engineering Company was hired by the GOL to manage the MPA. However, in 1964 Sanderson and Porter replaced Stanley engineering company. The GOL at the time preferred Stanley engineering company to carrying out the task of surveying, designing and supervising the Mount Coffee Hydroelectric project. //

With all of the LEC facilities damaged as a results of war, it became appropriate to effect the long awaited power system change, over which the years left Liberia as the only Country in Africa that operated power system base on North America standard of 60htz , 220/110v customer voltage.

In 1998, with funding with from the Danish Development Agency (DANIDA), a Danish Consulting firm NESA Team, carried out a power system conversion study. Today, Liberia has effectively converted its system from the North America standard to 50HTz 400/230V customer voltage.

Light, electricity, or current is necessary for the growth and development of any economy, especially the poor economies of Africa. Most people in these African countries do not have access to electricity. In 1996, the people of Sub-Saharan African countries had 28.4% access to electricity, and this access increased to 40.6% by 2021. In Liberia, this access increased from 3% in 1996 to 29.8% by 2021 (WB, 2023).

This access to electricity in Liberia is associated with costs, as no choice is without cost in any country or in any decision-making situation. High costs are associated with national decision-making in most African countries, with their respective money-driven decision-making situations. These situations are at once bad and very costly. They are bad because they are in the realm of bad governance. They are very costly because less costly choices could have been made. Less costly choices were not made and are not being made because the bad governance of state management remains corrupt. In the absence of electricity, most persons do not have access to schooling, health, food, and other basic needs.

Liberia is faced with three options in terms of access to electricity: two short to medium-term options and one long-term option. The first set of options come from the United States of America (USA) based company High Power Explanation (HPX) and the Turkish based company Karpowership. The long term option is from the CLSG Group of countries. HPX has a problem of access to the use of the railroad for transporting from ore from the Mifergui Mines from the Liberia-Guinea border to Liberia when the railroad is controlled by Arcelor Mittal, the world’s largest steel production company. All of the companies are profit-oriented. State management is Liberia is at once money-seeking and corrupt. The situation in Liberia forces State management to engage the first two companies because Liberia is seeking finance, even budgetary assistance. Yet, the State management is Liberia announces its preference for the CLSG option. What an irony!

A low power factor causes poor system efficiency. The total apparent power must be supplied by the electric utility. With a low power factor, or a high-kilovar component, additional generating losses occur throughout the system. //

The application of capacitor kilovars up to the no-load kilowatt-amperes results in a lagging power factor for all load conditions.

Look at the power triangle, kW kVA kVAR formula can be written as below,
kVA2 = kW2 + kVAR2

kVA = √ (kW2 + kVAR2)

Look at the above formula, the kVA is equal to the square root of the sum of the square of the kW and KVAR. //

kVAR is equal to the sin of power angle times of kVA.

kVAR = kVA * sin(φ)

or

Reactive power = apparent power * sin of power angle.
The power angle φ can be calculated by the cosine inverse of power factor //

kVAR is equal to the tan of kW. Hence the formula can be

kVAR = kW * tan (φ)

Reactive power in kilo volt-ampere reactive = kW * tan (power angle)

johnwalker 5d

nagle:
Norway is far enough along in this area that actuals are available.

Norway is, of course, an outlier both in its electrical generation and consumption per capita.

Around 95% of electricity generation in Norway is from hydroelectric power, and it is the largest producer of hydroelectric power in Europe. This is the result of a policy which has been in effect since 1892, and 90% of generation capacity is publicly owned.

Norway’s per capita electricity consumption is 24,182 kWh/year, ranking second in the world after Iceland (51,304 kWh/year). This is more than twice the U.S. at 11,267 kWh/year.

With abundant hydropower, electricity is the most common source for home heating and hot water, which has contributed to developing a grid which can support electric vehicle charging.

This isn’t to discount the value of the experience in Norway, where around 80% of new vehicle sales are electric, but their circumstances are unusually favourable to electric vehicles compared to countries without abundant base load hydroelectric power.

Mettelus > johnwalker 5d
From Euronews.com:

The number of fully electric cars in Norway exceeded 3 million in 2022, and the share of EVs among the total number of cars rose to 76 per 10,000 in 2021, up from only 2 per 10,000 in 2013.

Although new purchases are 80%, the total percentage of EVs is still very small. Successfully charging less than a percent of the vehicles is not a good indication of how it will go when 80% of the vehicles need to be charged. One car out of 100 can be charged at the bookstore.

Also, just a side note. In the McKinsey report they use EBITA and as Charlie Munger advised: Whenever you see EBITA, substitute BS. //

civilwestman 3d
I must wonder as to two practicalities in a place like Norway. According to a brief search, EV batteries lose 12 - 30 % of their range in cold weather - before the heater is turned on. Then, it drops around another 40%. I guess Norwegians just like the “cool” experience of gliding around in green vehicles. Are mink blankets an OEM option I wonder, like the Tsarist Russian troikas?

johnwalker

nagle:
Two islands with four chargers each can charge eight cars. Charging stations may be able to replace gas stations on the same real estate.

Current standards for electric vehicle charging stations have the following maximum power delivery:

• SAE J1772 DC Level 2 — 400 kW
• IEC 61851-1 — 80 kW
• Tesla NACS — 250 kW

(Again, these are maxima under the standards: many installed charging stations are lower power. A typical Tesla V2 Supercharger provides 120 kW.)

Plans for future higher power charging standards include the Megawatt Charging System 1 (MCS) with a rating of 3.75 megawatts (3000 amperes at 1250 volt DC).

Let’s compare this to a gasoline pump. A typical filling station pump in the developed world delivers around 50 litres per minute (38 l/min in Safetyland), and gasoline has an energy content of around 7500 kcal/litre depending on its formulation (around 5000 kcal/litre for pure ethanol and 8600 for #2 diesel). Plugging these into Units Calculator, we get:

(50 litres/minute) * (7500 kcal / litre) = 26.15 megawatt

so even the proposed MCS (which is primarily intended for large commercial vehicles and buses) delivers only around 1/7 the power of a gasoline pump.

Now, even getting installation of five megawatt electrical service is a pretty big thing in most places (that is the consumption of a very large office building), so it looks like building out an infrastructure which will allow electrical vehicle charging times competitive with gasoline filling will require very substantial upgrades to the power grid and local distribution facilities.

• 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.

Multi-functional power and energy meters meet ANSI C12.2 billing grade accuracy and are the ideal choice for the monitoring and controlling of power distribution systems. These revenue grade meters feature data logging capabilities, True-RMS parameter measurement, digital RS485 communication port, and are compatible with different current transformers.