Tuesday, December 18, 2012

permittivity (electric permittivity)


Permittivity, also called electric permittivity, is a constant of proportionality that exists between electric displacement and electric field intensity. This constant is equal to approximately 8.85 x 10-12 farad per meter (F/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized eo.
In engineering applications, permittivity is often expressed in relative, rather than in absolute, terms. If eo represents the permittivity of free space (that is, 8.85 x 10-12 F/m) ande represents the permittivity of the substance in question (also specified in farads per meter), then the relative permittivity, also called the dielectric constant er, is given by:
er = e / eo
e (1.13 x 1011)

Wednesday, December 12, 2012

Infra-Red Thermometer


The theory of operation 

All objects above absolute zero (-273.15°C) give ou t infrared energy. This radiated energy is emitted in all directions at the speed of light.

The instrument’s lens picks up this energy and focuses it on an infrared detector. This detector gives a voltage signal proportional to the quantity of energy received and hence proportional to the temperature of the object.
English

What is the  emissivity  that mean ??

Some objects not only give out infrared energy, but also reflect it as well. Unlike matt surfaces, brilliant or highly polished surfaces tend to reflect energy. This possible reflection is represented by a factor known as the emissivity, and can vary between 0.1 for a highly reflective surface, and 1 for a black body.


((TARGET DISTANCE / MEASUREMENT FIELD DIAMETER)) 
RATIO
This ratio, also known as the field of vision, indicates the diameter of the probes’ measurement field at a given distance from the target (see figure below).


In the case of a measurement point of small dimensions, it is important to bring the probe to a short enough distance from the target, so as to avoid including other points into the field of measurement.




Sunday, July 15, 2012

Definition of measurement categories


كثيرا ما نسال او نتسائل عن المعني الفني للتصنيف  ( cat IV او cat III )   كثيرا ما نواجها في المواصفات الفنية وغالبا ما ترفض مناقصات او تخرج من المنافسة لعدم مطابقتها للفئة المطلوبة 

الموضوع في غاية البساطة .. هذه الفئات ما هي الا تصنيف مراتب الحماية التي قد يتعامل معها الفني او الجهاز الكهربي المستخدم ويتم توضيحها كما يلي :-


1- Measurement category IV corresponds to measurements taken at the source of low-voltage installations. 

                Example: power feeders, counters and protection devices.

2- Measurement category III corresponds to measurements on building installations. 

             Example: distribution panel, circuit-breakers, machines or fixed industrial devices.

3- Measurement category II corresponds to measurements taken on circuits directly connected to low-voltage installations. 

        Example: power supply to electro-domestic devices and portable tools.

4- Measurement category I corresponds to measurements taken on circuits not directly connected to the network. 

Example: protected electronic circuits.



Wednesday, May 23, 2012

Operating principle of Circuit breaker

A circuit breaker as a switching and current interrupting consist of fixed and moving contacts, which are touching each other and carry the current under normal condition. When the circuit breaker is closed, the current carrying contacts, called the electrodes, engage each other under the pressure of a spring.


Whenever a fault occurs on any part of the power system, the trip coils of the breaker get energized and the moving contacts are pulled



The tripping circuit is a coil which can be energized and produce a magnetic field that attract an armature that can be able to handle with the mechanical system to open or close the contact of the circuit breaker

Sunday, April 15, 2012

What is a circuit breaker?

A circuit breaker is an apparatus in electrical systems that have the capability to, in the shortest possible time, switch from being an ideal conductor to an ideal insulator and vice versa. Circuit breakers requires to reduce arcing ie, Sparks jump for several inches in the highly ionized ambient atmosphere


TN-S system earthing



A TN-S system, shown in fig, has the neutral of the source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source, and the consumer’s earthing terminal is typically connected to the metallic sheath or armored of the distributor’s service cable into the premises.



Type of earthing systems


BS 7671 lists five types of earthing system: TN-S, TN-C-S, TT, TN-C, and IT.
T          = Earth (from the French word Terre)
N         = Neutral
S          = Separate
C         = Combined
I           = Isolated (The source of an IT system is either connected to earth through a deliberately introduced earthing impedance or is isolated from Earth. All exposed-conductive-parts of an installation are connected to an earth electrode.)

When designing an electrical installation, one of the first things to determine is the type of earthing system. The distributor will be able to provide this information. The system will either be TN-S, TN-C-S (PME) or TT for a low voltage supply given in accordance with the Electricity Safety, Quality and Continuity Regulations 2002. This is because TN-C requires an exemption from the Electricity Safety, Quality and Continuity Regulations, and an IT system is not permitted for a low voltage public supply in the UK because the source is not directly earthed. Therefore TN-C and IT systems are both very uncommon in the UK.

Monday, February 20, 2012

Power Quality Problems


The most common types of Power Quality problems are presented in the following Table


Wednesday, February 8, 2012

Power quality

What is Power Quality?
  1. The concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment.”
  2. “Within the industry, alternate definitions or interpretations of power quality have been used, reflecting different points of view.”
  3. The definition of Power Quality cannot be limited to the characteristics of the supply power. The definition must also include the requirements of the load.“


Power Quality Solution Strategy
  1. good grounding system 
  2. Harmonic solutions
  3. Surge solution 
  4. Voltage variation solution 

Friday, February 3, 2012

Grounding Codes and Standards


 Ideally a ground should be of zero ohms resistance. The NEC has stated that "A single electrode consisting of a rod, pipe, or plate which does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode..." Once you have added the supplemental ground you have met the requirement for the NEC.

This does not mean that the value of the ground now has to be 25 ohms or less. The ground resistance values objectives vary from industry to industry.

Telecommunications industry has often used 5 ohms or less as their value for grounding and bonding. The goal in ground resistance values is to achieve the lowest ground resistance value possible that makes sense economically and physically



Thursday, February 2, 2012

The need to ground


There are important reasons why a grounding system should be installed.

1- The most important reason is to protect people


2- Protection of structures and equipment from unintentional contact with live conductors


3- Ensure maximum safety from electrical system faults and lightning.

Wednesday, February 1, 2012

The meaning of earthing



It is a fundamental fact that electricity ALWAYS flows to the point of lowest potential. The task is to ensure that electricity, including faults, lightning and electronic noise, and flows to this point with maximum safety to people, while maintaining the reliability of equipment. Therefore we must ensure the safe, controlled flow of electricity with minimum voltage drop to earth in all cases.

The different between earth and ground 

  Ground:
The NEC, National Electrical Code defines a ground as: A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.


Earth:
The conductive mass of the earth, whose electric potential at any point is conventionally taken as equal to zero. (In some countries the term "ground" is used instead of "earth.")

Sunday, January 22, 2012

البداية



اولا ,,, تحية الي كل الدكاترة العظام الذين ساعدونا في مشوارنا العلمي والبحثي داخل الكلية وخارجها ، وكذلك تحية لكل زملائنا واخوتنا المهندسين والمهندسات الذين يدعموننا بالنصح والتوجيه حتى نصبح علي خير مثال ، شكرا لكم جميعا ، ونتمنى ان يوفقنا الله لرد الجميل 

ثانيا ,,, لقد اخذنا عهد علي انفسنا ، ان نبذل اقصى ما في وسعنا خدمة لزملائنا المهندسين ،، الذين يريدون التحول الي مجال المقاولات الكهربية ،، او حتى زملاء المستقبل ،، طلاب كليات الهندسة اصحاب مشاريع الــDistribution والتركيبات الكهربية 

ثالثا ،،، وعليه ،  نحاول ان نسعى جاهدين لتقديم افضل خدمة لكافة الزملاء ... وذلك بتقديم اخر التطورات والتحديثات في مجال الاستشارات الهندسية والتصميمات الكهربية والمقاولات .. واتاحة الفرصة لكافة المهندسين بالمشاركة والاطلاع علي اخر الدوريات العلمية والمواصفات القياسية العالمية ،،، وذلك من خلال نشرها   اول باول علي هذه المدونة .

ومن يريد التدريب العلمي والعملي علي مشروع حقيقي ، نقدم له دورتنا المتخصصة في مجال التصميمات والتركيبات الكهربية والتي يتم فيها مراعاه استخدام كافة البرامج الهندسية الحديثة والمتطورة ، فهدفنا هو ان يصبح المهندس قادرا علي تصميم النظم الكهربية بالمنشأت الحديثة باستخدام البرامج الهندسية المتطورة وعلي دراية كبيرة بطرق التركيبات الكهربية والتجهيزات الهندسية المختلفة لذلك يسرنا ان نقدم لكم دورة التركيبات والتصميمات الكهربية بالمباني 

فمعنا سيتم التدريب علي كافة مراحل التصميم الكهربي والاطلاع علي طرق تنفيذ كل مرحلة من هذه المراحل ، حيث يتم التدريب علي كافة ( اعمال الانارة – القوي – التكييفات – الطوارئ – احمال التيار الخفيف كالتليفوانات والانترنت ، تقدير الاحمال ، اختيار الكابلات واختيار القواطع الكهربية ،اختيار المحولات ) بالاضافة لعمل الحسابات الكاملة لكل مرحلة ومطابقتها مع المواصفات العالمية الموصي للعمل بها في هذا المجال .