How to Check for Electrical Cable Insulation Faults

Good cable insulation has high resistance; poor insulation, relatively low resistance and cable faults can show very low resistance or a zero insulation indicating a short circuit and/or fault.. The actual resistance values can be higher or lower, depending upon on the temperature or moisture content of the insulation  

With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative and an indication if the cable has a fault.

To check the cable insulation values we commonly use The Megger insulation tester which is a small, portable instrument that gives you a direct reading of insulation resistance in megohms. For good insulation, the resistance usually reads in the high megohm range.

The Megger insulation tester is a high-range resistance meter (ohmmeter) with a built-in direct-current generator. This type of meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied. The Megger tester is non- destructive; it does not cause deterioration of the cable insulation.

The generator is often line or battery-operated to develop a high  DC voltage which causes a small current through and over the surfaces of the insulation being tested. This current (usually at an applied voltage of 500 to 1000 volts or more) is measured by the meter, which has an indicating scale. which reads increasing resistance values from  0 up to infinity,

Pressure Variations in a High Rise Building due to Elavators

A=Single Car Shaft                            

 B = Double Car Shaft
Click on image to enlarge 
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Basic Capture Hoods

Local exhaust which results in capture effect is such that the hood utilized; captures, contains or receives contaminates generated by the local source. The hood generates the capture effect by converting duct static pressure to velocity pressure and hood losses (e.g., slot and duct entry losses). The concept is described by the following formula:(Also See NRC Publication's #29, #31 and #84 at www.nrctraining.com)

Hood loss (HL) is equal to:EQ (1) HL = K x VP = {SP(h)} = VP Where K = loss factor
VP = Velocity pressure in exhaust duct ,{SP(h)} = Absolute static pressure approximately 4 to 6 duct diameters upstream from the hood entrance.

The capture hood's ability to convert this static pressure to velocity pressure is given by the hood's coefficient of entry C(e). This is further defined as follows:
EQ (2) C(e) = Q(ideal)/Q(actual) = divide VP/SP(h) = divide 1/(1+K)

Any time you enclose the material giving off the contaminate emissions, you will be able to greatly reduce the amount of air required to produce the required capture velocity. You must always keep the source of contaminate emissions as close to the local hood as possible. The hood must also be designed to allow a smooth entrance of air into the hood so that all of the air entering the hood will be capable of capturing the contaminates. The idea of the local exhaust system is to prevent worker inhalation of contaminates.

For this reason, the hood has to be located so that it does not cause the contaminates to move through the occupant's and/or worker's breathing zone in order to make its way to the hood entrance. This is especially true if the hood is to provide protection to workers leaning over an operation which involves utilizing an open surface tank or welding bench.

For a lot more detailed information you may want to review my book "Contamination Control Ventilation" Or send me an email if you just want to
explore some thoughts. Learn more

Thanks

Hal

Magnetic vs. Electronic Ballasts

If an existing lighting system is to be evaluated for changes it will be important to determine if the system is utilizing magnetic or electronic ballasts.

The full output electronic ballast is a high frequency version of the conventional magnetic "core-and coil" ballast. The electronic ballast operates fluorescent lamps more efficiently at frequencies greater than 20,000 Hz.

The full output electronic ballast is rated with a ballast factor of at least .85. This factor actually identifies the output of light from the ballast-lamp combination. The ballast factor is simply that percentage of the lamp's rated lumens actually produced by the ballast lamp combination.

Magnetic ballasts normally have a ballast factor of between .90 to .95. The electronic ballast however can be purchased in a large range of ballast factors. You can purchase an electronic ballast that may range from 1.00 to 1.30 which acts as a booster with the lamp and actually lets the lamp produce a greater amount of lumens then the lamp is actually rated for.

On the other hand, you could purchase an electronic ballast with a range of .45 to .85, which shows that some ballasts can be utilized to actually reduce the amount of light put out by the fixture. If the ballast is a full output type, it would have a ballast factor that would exceed .85.

A partial output electronic ballast is utilized to have a lighting fixture put out less light than that which it is rated. This can be useful for several different types of installations.

A simple way to determine if the installed ballast is electronic or magnetic is to utilize a "strobe top". Some ballast manufacturers supply these free of charge to designers and installers. The top is simply spun directly under the fixture in question. If you see pattern lines, the lights are operating at 60 Hz and therefore are utilizing magnetic ballasts. If the pattern turns out to be a smooth pattern with no lines, the fixtures are utilizing high-frequency electronic ballasts.

Elevators and other Non-Linear Load problems for Generators

For emergency generator systems utilized for life safety the elevators may be the largest non-linear load. As important as these are the designer must make sure that these non-linear loads are not going to create a great deal of stress for the generators. In some cases if elevators are left to operate off the generator for a long duration they may cause the generators to burn out.

When designed to operate for standby power, the emergency generator must be capable of operating the elevators safely and with a great deal of reliability. If the elevators do not operate properly while on emergency power or if the generator can not handle the elevator load for a long duration serious problems may occur.

The most commonly ignored operation of an emergency generator is its ability to handle the elevator loads safely and over a long duration. Especially when the emergency generator system is designed for short duration power outages, let’s say, a class two system. If in fact the generator capacity was selected on a peak load expected to be occurring for short durations, say two hours, but instead the owner try’s to operate the emergency generator for a long period of time say, 48 hours at peek load, the generator may burn out.

Additionally, the owner must check to see if the local codes require elevator standby power testing annually. If more than one elevator is capable of running simultaneously, the local codes may require them to all be tested simultaneously.
The owner operator must also realize that with the advent of SCR and VFD drives for elevators and other devices, existing emergency generators may not be capable of providing the proper power to operate the elevators in an emergency over a long period of time. Additionally, older emergency generators may have a difficult time with the current demand changes that solid-state drives require.

With emergency generator systems current and voltage harmonics differ greatly from that produced by utility power. Improper grounding methods and increased impedance of the generator system can cause additional problems with sensitive devices on the emergency feeder system, due to increased harmonics and RFI. Studies and testing have shown that problems as follows can develop while operating elevators on emergency power:

The Total Harmonic distortion can increase substantially over that expected from utility power.

Grounding can be found to be insufficient.

Voltage regulation as the elevators operate can be found to vary by up to 15 to 25%.

Voltage may dip to values not tolerated by solid state drives.

It is very important that when upgrading to a modern solid state elevator drive that the owner also upgrade older emergency generators that they may be considering to operate all or any of the elevators in the event of a power disruption. If the emergency generator is not to be upgraded than an alternate type of motor drive may have to be specified.

In evaluating the emergency generator-elevator relationship other items effecting the generator operation may also have to be evaluated. Such items that could effect the operation of the generator and how it responds to the elevators are as follows:

What else is operating off the emergency system? Variable frequency drive motors?

Are any sensitive UPS systems expected to be served by the generators?

Are radio and emergency telephone services to be operated from the generator and if so how are they shielded and grounded?

Is the existing generator able to handle the regenerative power from the elevator SR and not have its performance adversely affected?

How will the emergency generators voltage regulator be effected and can it handle the major voltage variation it may be subject to?

Which type of elevator drive is to be installed and how will it effect the generator in actual emergency operation situations. The various types of common drives that may be utilized are as follows:

12 pulse SCR drives with out filters.
Variable Frequency-variable Voltage AC type Non Regenerative.
Motor Generator Set.
Six Pulse SCR drive with filters.
Hydraulic(Across the line)

The Desalination Process

Florida and many other coastal states are moving towards Desalination to supply some of their fresh water needs. Though there are many different desalination processes this review details some of the more popular methods utilized today.

Maintenance and operations for many of these systems can be extensive. On several High Temperature Distillation systems I have worked on the M & O costs were so extensive that efforts to keep them under control became all consuming. On reverse osmosis systems we had to keep very close control of all pumping costs. The following goes into more detail.

Process limitations. The various desalination processes presently available have limitations that must be considered prior to selecting a desalination process for a particular site. These limitations apply only to the desalination processes themselves; pretreatment can be and is often used to bring a saline feed water within limits so that a desalination process can be used. The raw feed water chemistry for all desalination systems must be evaluated thoroughly for constituents that may precipitate in the desalination system.

a. High-temperature distillation. High-temperature distillation is limited by the saturation of alkaline earth metal salts, such as CaSO4, BaSO4, SrSO4, CaCO3, BaCO3, and SrCO3. Carbonate salt scaling can be controlled by acid addition. The recovery of water from a high temperature distillation plant is usually limited by calcium sulfate solubility. When the concentration of the sulfate and the limiting alkaline earth metal is one third of the saturated condition at ambient temperature, distillation design must include pretreatment to reduce or inhibit the scaling ions. High-temperature distillation is also limited to oil and grease levels below 1 milligram per liter. All other limitations on the high-temperature distillation process are equipment specific and require individual evaluation.

b. Low-temperature and mechanical distillation. Low-temperature and mechanical distillation systems are limited to operation below saturation of alkaline earth sulfates and carbonates. The lower operating temperature permits economical operation on waters that are at or below half saturation at ambient temperature. Oil and grease are limited to less than 1 milligram per liter. Any other limitations are equipment specific.

c. Reverse osmosis. The most severe limitation on reverse osmosis is the maximum limit of 50,000 milligrams per liter of total dissolved solids in the feed water. Another limitation is that there must be no iron in the feed water. This limitation is so rigid that only stainless steel and nonferric materials will be used downstream of the iron removal. The solubility of alkaline earth sulfates and carbonates limits reverse osmosis treatment. Any water containing less than 4,000 milligrams per liter of total dissolved solids that would be saturated with an alkaline earth sulfate when the concentration is multiplied by 1.5 should not be considered for reverse osmosis desalination. Reverse osmosis is limited to waters that do not have silica saturation in the reject brine. Silica chemistry is extremely complex. When the molybdenum reactive silica concentration exceeds 30 milligrams per liter as SiO2 or the pH exceeds 8.3 in the brine stream, an environmental chemist or engineer should be consulted. Reverse osmosis is also limited to the treatment of waters with less than 1 milligram per liter of oil and grease.

(1) Cellulose acetate membranes. Cellulose acetate membranes are usually limited to pH levels between 4.0 and 7.5. Cellulose acetate membranes require some form of continuous disinfection with the feed water to prevent microbial degradation of the membranes and can tolerate up to 1 milligram per liter of free chlorine. Therefore, cellulose acetate membranes are usually disinfected by maintaining 0.2 to 0.9 milligrams per liter of free chlorine in the feed water. Cellulose acetate membranes cannot be used on waters where the temperature exceeds 88 degrees Fahrenheit. Cellulose acetate membranes should not be used at pressures greater than the manufacturer's recommended pressure, since they are prone to membrane degradation by pressure compaction.

(2) Polyaromatic amide membranes. Brackish water polyaromatic amide membranes are generally limited to operation in feed waters between pH 4 and pH 11. Polyaromatic amide membranes are less pH tolerant and should not be used outside of the range pH 5 to pH 9. All polyaromatic amide membranes are limited to use on feed streams that are free of residual chlorine. If chlorination is necessary or desirable as a pretreatment option, complete dechlorination must be effected. Polyaromatic amide membranes are tolerant of water temperatures up to 95 degrees Fahrenheit. While polyaromatic amide membranes are not as
quickly or completely compacted as are cellulose acetate membranes, manufacturer's recommended pressures must be followed to prevent mechanical damage to membrane modules.

d. Electrodialysis reversal. While electrodialysis reversal has been used to treat water as saline as sea water, 4,000 milligrams per liter of total dissolved solids is considered to be an upper limit for economical operation. Some electrodialysis membranes can tolerate strong oxidants, like chlorine, but most cannot. The reversal of polarity used in electrodialysis reversal for removal of scale allows operation on water that is saturated with alkaline earth carbonates. Saturation with an alkaline sulfate with low carbonate alkalinity should be avoided.