How to locate underground faults on cable?

A  common approach to test cable and determine insulation integrity is to use a Hi-pot test. In a hi-pot test, a DC voltage is applied for 5 to 15 min. IEEE-400 specifies that the hi-pot voltage for a 15-kV class cable is 56 kV for an acceptance test and 46 kV for a maintenance test (ANSI/IEEE Std. 400-1980). Other industry standard tests are given in (AEIC CS5-94, 1994; AEIC CS6-96, 1996; ICEA S-66-524, 1988). High-pot testing is a brute-force test; imminent failures are detected, but the amount of deterioration due to aging is not quantified (go/no-go test).


The DC test is controversial – some evidence has shown that hi-pot testing may damage XLPE cable (Mercier and Ticker, 1998). EPRI work has shown that dc testing accelerates treeing (EPRI TR-101245, 1993; EPRI TR-101245-V2, 1995).

For Hi-pot testing of 15-kV, 100% insulation (175-mil, 4.445-mm) XLPE cable, EPRI recommended:

• Do not do testing at 40 kV (228 V/mil) on cables that are aged (especially those that failed once in service and then are spliced). Above 300 V/mil, deterioration was predominant.
• New cable can be tested at the factory at 70 kV. No effect on cable life was observed for testing of new cable.
• New cable can be tested at 55 kV in the field prior to energization if aged cable has not been spliced in.
• Testing at lower dc voltages (such as 200 V/mil) will not pick out bad sections of cable.

Another option for testing cable integrity: ac testing does not degrade solid dielectric insulation (or at least degrades it more slowly). The use of very low frequency AC testing (at about 0.1 Hz) may cause less damage to aged cable than DC testing (Eager et al., 1997) (but utilities have reported that it is not totally benign, and ac testing has not gained widespread usage).
The low frequency has the advantage that the equipment is much smaller than 60-Hz AC testing equipment.

Fault Location

Utilities use a variety of tools and techniques to locate underground faults. Several are described in the next few paragraphs [see also EPRI TR-105502 (1995)].

Divide and conquer

On a radial tap where the fuse has blown, crews narrow down the faulted section by opening the cable at locations. Crews start by opening the cable near the center, then they replace the fuse. If the fuse blows, the fault is upstream; if it doesn’t blow, the fault is downstream.
Crews then open the cable near the center of the remaining portion and continue bisecting the circuit at appropriate sectionalizing points (usually padmounted transformers). Of course, each time the cable faults, more dam-age is done at the fault location, and the rest of the system has the stress of carrying the fault currents. Using current-limiting fuses reduces the fault-current stress but increases the cost.

Fault indicators

Faulted circuit indicators (FCIs) are small devices clamped around a cable that measure current and signal the passage of fault current. Normally, these are applied at padmounted transformers. Faulted circuit indicators do not pinpoint the fault; they identify the fault to a cable section.

Figure 1 - Typical URD fault indicator application

After identifying the failed section, crews must use another method such as the thumper to precisely identify the fault. If the entire section is in conduit, crews don’t need to pinpoint the location; they can just pull the cable and replace it (or repair it if the faulted portion is visible from the outside). Cables in conduit require less precise fault location; a crew only needs to identify the fault to a given conduit section.

Utilities’ main justification for faulted circuit indicators is reducing the length of customer interruptions. Faulted circuit indicators can significantly decrease the fault-finding stage relative to the divide-and-conquer method. Models that make an audible noise or have an external indicator decrease the time needed to open cabinets. Utilities use most fault indicators on URD loops. With one fault indicator per transformer (see Figure 1), a crew can identify the failed section and immediately reconfigure the loop to restore power to all customers. The crew can then proceed to pinpoint the fault and repair it (or even delay the repair for a more convenient time).

For larger residential subdivisions or for circuits through commercial areas, location is more complicated. In addition to trans-formers, fault indicators should be placed at each sectionalizing or junction box. On three-phase circuits, either a three-phase fault indicator or three single-phase indicators are available; single-phase indicators identify the faulted phase (a significant advantage). Other useful locations for fault indicators are on either end of cable sections of overhead circuits, which are common at river crossings or under major highways. These sections are not fused, but fault indicators will show patrolling crews whether the cable section has failed.

Fault indicators may be reset in a variety of ways. On manual reset units, crews must reset the devices once they trip. These units are less likely to reliably indicate faults. Self-resetting devices are more likely to be accurate as they automatically reset based on current, voltage, or time. Current-reset is most common; after tripping, if the unit senses current above a threshold, it resets [standard values are 3, 1.5, and 0.1 A (NRECA RER Project 90-8, 1993)]. With current reset, the minimum circuit load at that point must be above the threshold, or the unit will never reset. On URD loops, when applying current-reset indicators, consider that the open point might change.


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Grounding and bounding for electrical systems

Why is Good Grounding Important?

The transient nature of lightning with its associated fast rise times and large magnitude currents mean that special consideration needs to be given to grounding, for lightning protection to be effective.

Many factors such as soil resistivity variations, installation accessibility, layout and existing physical features are all site specific and tend to affect decisions on earthing methods employed.

The primary aim of a direct strike grounding system is to efficiently dissipate lightning energy into the ground and to help protect equipment and personnel.

Earthing Principles

Low impedance is the key to lightning protection. All earthing connections should be as short and direct as possible to minimize inductance and reduce peak voltages induced in the connections. The ground electrode system must efficiently couple lightning surges into the ground by maximizing capacitive coupling to the soil.

The resistance of the ground itself to lightning currents must also be minimized. Only when all these factors are taken into account will maximum lightning protection be achieved.

 

Ground Impedance

Soil resistivity is an important design consideration. It varies markedly for different soil types, moisture content and temperatures and gives rise to variations in ground impedances.
The voltage generated by a lightning discharge depends primarily on the risetime of the current and the impedance (primarily inductance) of the path to ground. Extremely fast rise times result in significant voltage rises due to any series inductance resulting from long, indirect paths, or sharp bends in the routing of ground conductors.

This is why short, direct ground connections are important.

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Industrial Control Wiring Guide

Wires and preparation for control wiring

Electrical equipment uses a wide variety of wire and cable types and it is up to us to be able to correctly identify and use the wires which have been specified. The wrong wire types will cause operational problems and could render the unit unsafe.

Such factors include:

• The insulation material;
• The size of the conductor;
• What it’s made of;
• Whether it’s solid or stranded and flexible.

These are all considerations which the designer has to take into account to suit the final application of the equipment.

A conductor is a material which will allow an electric current to flow easily. In the case of a wire connection, it needs to be a very good conductor. Good conductors include most metals. The most common conductor used in wire is copper, although you may come across others such as aluminium. An insulator on the other hand is a material which does not allow an electric current to flow. Rubber and most plastics are insulators.

Insulation materials

Wires and cables (conductors) are insulated and protected by a variety of materials (insulators) each one having its own particular properties. The type of material used will be determined by the designer who will take into account the environment in which a control panel or installation is expected to operate as well as the application of individual wires within the panel.


As part of the insulating function, a material may have to withstand without failing:

• Extremes of current or temperature;
• A corrosive or similarly harsh environment;
• Higher voltages than the rest of the circuit.

Because of these different properties and applications, it is essential that you check the wiring specification for the correct type to use.





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9 Reasons for Automation of Manufacturing Processes

Manufacturing automation

Automated manufacturing systems operate in the factory on the physical product. They perform operations such as processing, assembly, inspection, or material handling, in some cases accomplishing more than one of these operations in the same system.

They are called automated because they perform their operations with a reduced level of human participation compared with the corresponding manual process. In some highly automated systems, there is virtually no human participation.

Companies undertake projects in manufacturing automation and computer-integrated manufacturing for a variety of good reasons. Some of the reasons used to justify automation are listed below. Of course, there are many other reasons, so feel free to add your reasons below in the comment box.
Also, I’ve put few interesting videos with latest news in automation of manufacturing processes at the bottom of this article. Enjoy!

1. To increase labor productivity

Automating a manufacturing operation usually increases production rate and labor productivity. This means greater output per hour of labor input.

2. To reduce labor cost

Ever-increasing labor cost has been and continues to be the trend in the world’s industrialized societies. Consequently, higher investment in automation has become economically justifiable to replace manual operations.

Machines are increasingly being substituted for human labor to reduce unit product cost. While this is not good for people, it’s good enough for production. Sad but true, isn’t it?

3. To mitigate the effects of labor shortages

There is a general shortage of labor in some countries, and this has stimulated the development of automated operations as a substitute for labor.

4. To reduce or eliminate routine manual and clerical tasks

An argument can be put forth that there is social value in automating operations that are routine, boring, fatiguing, and possibly irksome. Automating such tasks serves a purpose of improving the general level of working conditions.



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8 tips to avoid ground loops when dealing with signal level circuits

Ground loops and signal noise

General recommendation is to properly design and implement the facility’s grounding system to avoid unwanted involvement of ground loops with the operation of the equipment. This kind of approach can also eliminate the need to consider equipment modifications and to engage in costly diagnostic efforts since most trouble involving common-mode noise is avoided in the signal circuits.

It is generally not possible in complex systems with interconnected data and signal conductors to avoid all ground loops.

Some eight tips that may be used to avoid the detrimental effects of such ground loops include:


Tip #1
Where possible, cluster the interconnected electronic equipment into an area that is served by a single signal reference grid (SRG). If the interconnected equipment is located in separate, but adjacent rooms, then a common signal reference grid should serve all the rooms.

Tip #2
Effectively bond each frame/enclosure of the interconnected equipment to the SRG. In this way, the SRG acts like a uniformly shared ground reference that maintains a usefully low impedance over a very broad range of frequency. Typically, from dc to several tens of MHz, for example.

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Basics of Sensors

Sensors

One type of feedback frequently needed by industrial-control systems is the position of one or more components of the operation being controlled. Sensors are devices used to provide information on the presence or absence of an object.
Siemens sensors include limit switches, photoelectric, inductive, capacitive, and ultrasonic sensors. These products are packaged in various configurations to meet virtually any requirement found in commercial and industrial applications. Each type of sensor will be discussed in detail.
At the end of the course an application guide is provided to help determine the right sensor for a given application.

Technologies

Limit switches use a mechanical actuator input, requiring the sensor to change its output when an object is physically touching the switch.

Sensors, such as photoelectric, inductive, capacitive, and ultrasonic, change their output when an object is present, but not touching the sensor.

In addition to the advantages and disadvantages of each of these sensor types, different sensor technologies are better suited for certain applications. The following table lists the sensor technologies that will be discussed in this course.

Contact Arrangement

Contacts are available in several configurations. They may be normally open (NO), normally closed (NC), or a combination of normally open and normally closed contacts. Circuit symbols are used to indicate an open or closed path of current flow. Contacts are shown as normally open (NO) or normally closed (NC).

The standard method of showing a contact is by indicating the circuit condition it produces when the contact actuating device is in the deenergized or nonoperated state.

For the purpose of explanation in this text a contact or device shown in a state opposite of its normal state will be highlighted. Highlighted symbols used to indicate the opposite state of a contact or device are not legitimate symbols.

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