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Wirings / Cables

  • Cable Glands

· Cable will be terminated by a cable gland before going into a junction box/equipment. The purpose of the cable gland is

· to grip the cable so that it hangs on to the equipment

· to void water ingress into the junction box

· To ensure hazardous are protection

The usual cable entry used is M20 type. The M’s range from M16 to M175 (smaller to larger cables). The complete M20 annotation is usually described as ‘M20x15’. 15 here means 15 mm length of the thread. M20 usually has a hexagon ring to ensure better gripping

  • Cable Gloves
    • After the cable gland, the cable will usually be covered by a cable glove. This is for cosmetics and also to avoid direct hit of rain on the cable gland.
  • Cable routes
    • Electrical and Instrument cables shall be 3m apart with exception of crossing. As long as the instrument cable is crossing perpendicularly with electrical cables, this shall have no effect. When crossing it should be at least >0.3m apart
    • Instrument cables should not be routed through severe EMI areas
    • If possible, cables should be routed though areas that are protected by lightning strikes
    • Cables should be routed near earthed steel structures and piping to reduce EMI
    • Transmission from field junction box to MDF must go underground in a multicore cable to prevent damage from fire or mechanical forces and also to prevent theft.
  • Cable Properties

· Cable thickness

· Cable thickness will determine the maximum cable carrying capacity.

· When one says cable thickness, it means the inner core NOT including the insulation.

· Cable thickness is usually measured in mm2 and will range between 0.005 to 100 mm2

· Sometimes cable thickness is measured in mm and this will range between 0.08 to 11 mm (you can calculate the area by using [PI*(D/2)^2]

· Field Instrument Signal wires

· use copper type wires which are usually around 1.13 mm

· The minimum cable diameter should be 1.13mm for single pair wires and 0.8mm for multicore wires (PTS)

· When specifying cable thickness, one will use standard. The most common standards used are

· American Wire Gauge (AWG)

· Standard Wire Gauge (SWG)

· Cable Capacitance

·

· Cable Insulation

· Cable insulation will determine the maximum voltage carrying capacity

· Screened or Unscreened

· Also called metal sheet. This is to remove noise and is made up of aluminum

· Metal sheet layer must be provided for small voltage signals to remove noise (Note that this wire must be grounded to instrument earth at the MDF). It is usually made of aluminum. TSometimes the metal screen will be also traced by a thin copper wire. This is to ensure continuity of the metal screen since usually, when the wires are bent, the aluminum sheath may tear apart. This wire is also called a drain wire

· Individual or overall screen

· Signal cables require screen while power cables do not

· The metal screen maybe available on every pair of cables. Even in a 10P-multicore cable, each pair will have a metal screen.

· Some multicore cable provide an single overall screen wire and do not have single wires on each pair

· Paired or Unpaired

· Some cables are paired while some aren’t.

· When they are paired, it is common to have individual screen for each pairs

· Inner layer insulating Material

· Typically polyethylene is used

· Outer layer insulating material

· Typically PVC is used because it is fire proof

· Underground Cable Protection

· Underground cabling must be provided with a special layer called AL/HDPE/PA (aluminium with high density polyethylene with polyamide (nylon) layer) to prevent moisture, chemical fumes or termite attacks. The AL (aluminum) layer also acts as a metal screen, hence no MS is required when AL/HDPE/PA is used

· Mechanical Protection

· Some cables will have an armour Layer

· It can either be steel wired armour or steel wire braiding

· To prevent mechanical forces, the use of steel wire armored or steel wire braiding is required (Note that this wire must be grounded to safety earth and the JB). Steel wire braiding is like a net style interface. This wire can also act as a metal screen. In this case the wire is grounded to the clean earth at the MDF. However some protective is installed at the cable gland of the JB so that the braided steels do not touch the gland since this gland is grounded to safety earth

· Fire Protection

· Fire resistant cables have a special layer before the SWA called ‘MICA’ a silica based material having high fire resistance.

· Fire retardant cables will have a high oxygen index. Oxygen index refers to how much oxygen is required to burn the cables. Normal cables will have oxygen index of 23% while fire retardant cables will have o2 index of 30%

· Thermocouple Cables

· If for thermocouple we require type K (Chromel Alumel) connections, which are red(+ve) and yellow(-ve) of color

  • Cable Construction Guide:-

· As a general guideline, instrument cables must have the following construction:-

· The conductor (Usually Copper Type)

· Insulator for conductor (Usually Poly Ethlyene)

· A Bedding Layer (To avoid direct contact of armour layer on screen) This layer provides insulation on the effect circulating ground currents (Since electrical earth is grounded in many places, current will flow in the ground wire due to ground potential difference)

· Cables shall be specified with low smoke and zero halogen. Halogen is very toxic to the body

  • PTS outlines a signal level class level from 1 to 5. Level 1 is for computer us signals, thermocouples and analyzers. Instrument 4-20mA signals are Level 2. Level 4 and 5 are for power cables.
  • KR2 Project instrument wire specs which are available in the market are of the following

· 1Pair/2P/3P/1Triad/2T/3T - 1.13mm PEI-MS -SWA-PVCS

· 1Pair/2P/1T/2T/3T – 1.13mm PEI-MS-AL/HDPE-SWA-HDPE/NC (for underground)

· 5P/10P/20P/10T/15T – 0.8mm PEI-MS-AL/HDPE-SWA-HDPE/NC (for underground)

· 1P/2P – 4.0mm2 PEI-MS-AL/HDPE-SWA-PVCS (for instrument power cables)

· 1P/5P/10P – 4.0mm2 PEI-MS-AL/HDPE-SWA-HDPE/NC (for instrument power cables)

· 1P/2P – 1.13mm PEI-MS-PVCS (internal cabinet wiring or installations with conduit)

· 10P/20P – 0.8mm PEI-MS-PVCS (internal cabinet wiring or installations with conduit)

· Fibre Optic Cables – FO-AL/HDPE-SWA-PVCS/NC (for underground)

· Cable Costs bought from vendor (who already marked up the price) in general are more or less the following

· 1P – RM40/m

· 10P – RM120/m

· Fibre Optics

  • Multi Mode

· Cheaper, more commonly used

· Has a larger diameter core

· 100 MBit/s for 2KM, 1000 MBit/s for 500m, 10GBit/s for 300m

· Multiple Data, Have 2 different wavelengths

  • Single Mode – Only 1 wavelength

· Cat 5 Cables

  • In the US the quality of cables to carry out signal is defined by CatX. Cat5 cable is currently the recognized cable for broad band transmission. Cat5e cable is the cable being recommended by IEEE and many equipment manufacturers. In the Europe they are rated in class, Class D being simlar to cat5 cables
  • CAT5 is an Ethernet cable standard defined by the Electronic Industries Association and Telecommunications Industry Association (commonly known as EIA/TIA). CAT5 is the 5th generation of twisted pair Ethernet cabling and the most popular of all twisted pair cables in use today. It has 4 twisted pairs of copper wire terminated on an RJ45 connector. Cat-5e (CAT 5 enhanced) cabling supports frequencies up to 100 Mhz and speeds up to 1000 Mbps.

· Color Coding

  • The purpose of cable color coding is to know

·

Junction Box

· Cable entries shall be from the bottom. This is to prevent water ingress. If there is not enough space at the bottom, installation can be from the side. The cables however ned to be slanted to the bottom to allow water to flow downwards

· Ex junction boxes have a groove to allow heat to dissipate through. This will allow heat to go out of the junction box. CAUTION, it is wrong to tape the junction box as it will seal this groove. Experience has shown that when this groove is sealed, we may get melted termination blocks

·

Wired Signal transmission

  • Signal transmission in wires are exposed to the following 3 types of noise

· Magnetically induced noise.

· This is caused due to flux produced at the flowing current. When another wire is nearby, it will cause a flux cutting and induce voltage.

· Using a twisted pair wire will cancel out this induction

· Static noise.

· This is caused since two wires are close by, the wires create and imaginary capacitance between. The capacitance causes a build up of charge.

· By having a metal screen wire, this charge can be dissipated to clean earth (instrument earth)

· Common mode noise.

· This is the noise caused from the current created by different electrical grounding points

·

  • The electromagnetic spectrum are as follows:-

Wireless Transmission

  • Wireless transmission is performed by a medium called the electromagnetic spectrum. Currently only the radio and microwave frequencies are used for this communication
  • The electromagnetic spectrum are as follows:-

· 1 to 30 to 300Hz

· Extremely Low Radio Frequency (ELF) and Super low frequency (SLF)

· Used for Communication with submarines – This is a one way communication

· Waves are produces by mounting 2 large antennas apart

· Extremely slow communication, Few characters per minutes

· 300-3KHz – Ultra Low Radio Frequency (ULF) – Used for communication in mines

· 3 - 30KHz

· Very Low Radio Frequency (VLF)

· Can penetrate up to 40m deep under water

· Used for communication with satellites on the surface

· Used in electromagnetic geophysical surveys (detecting gold, minerals)

· 30 – 300KHz

· Low Radio frequency (LF)

· Used for navigation, radio clocks

· In some countries used for AM modulation

· 300 – 3000KHz

· Medium Radio Frequency (MF)

· Used for AM Broadcasting

· 3 - 30MHz

· High Radio Frequency (HF)

· Ionosphere can reflect these frequencies – Due to this HF has a very long distance. This phenomena is called skywave. Skywave is however effected by a lot of whether factors

· Used by amateur radios (private radios) since they would enjoy the long transmission

· 30 – 300MHz

· Very High Frequency (VHF)

· FM Radio transmission

· Previously used for black and white Television Broadcast. Could not be used for colour TV due to limited bandwidth

· 300MHz – 3GHz

· Ultra High Frequency (UHF)

· 450MHz is used for ATUR communication

· 900 – 1800MHz is used for Cellular Communication

· 900 Mhz band is used for Celcom and Maxis communication

· 1800 Mhz is used for adam.timecel and digi

· Wireless transmitters use 902 – 928 MHz Signal

· 470 – 800 MHz is used for color TV channels

· 2.4GHz Wifi Communication

· 2.45 GHz is used for microwave oven and blue tooth

· 3 – 30GHz

· Super High Frequency (SHF), popularly called the Microwave

· Used for WiMax

· Used for radar, as most objects reflect microwave signals

· Blue tooth use

  • As seen above, most signals are segregated by frequency. A recent technology, CDMA (Code Division Multiple Access) now allows signal to be segregated by code. CDMA is used for frequencies constructed from the band between
  • ISM (Industrial, scientific and medical) Band is a defined by ITU-R for the use of Industrial, Scientific and medical purposes which is free. All communication equipment must accept any interference from ISM equipment

· 6765-6795 kHz (centre frequency 6780 kHz)

· 13553-13567 kHz (centre frequency 13560 kHz)

· 26957-27283 kHz (centre frequency 27120 kHz)

· 40.66-40.70 MHz (centre frequency 40.68 MHz)

· 433.05-434.79 MHz (centre frequency 433.92 MHz) in Region 1

· Walkie Talkie

· 902-928 MHz (centre frequency 915 MHz) in Region 2

· Wireless Transmitters

· Cordless Phones

· 2400-2500 MHz (centre frequency 2450 MHz)

· 5725-5875 MHz (centre frequency 5800 MHz)

· 24-24.25 GHz (centre frequency 24.125 GHz)

· 61-61.5 GHz (centre frequency 61.25 GHz)

· 122-123 GHz (centre frequency 122.5 GHz)

· 244-246 GHz (centre frequency 245 GHz)

  • Wireless signals are created by an Antenna which generates a radiating EM field
  • The government body which dictates frequency usage in Malaysia is the MCMC (Malaysian Communication and Multimedia Commission). We pay them yearly
  • About Handphones

· HP use a network called the cellular network. The cellular network is made out of cellular towers, normally known as cell sites. The most popular cellular network standard use is GSM(Global System for Mobile Communication)

· GSM operates in 2 frequency bands 900Mhz and 1800Mhz

· The first generation of HP, called 1G, uses analogues signals for communication.

· The second generation 2G, uses digital signals for communication. Voice signal is sent in a digital signal of around 10Kbits/s

· The third generation of mobile phones is 3G. This was enabled by packet switching technology which provides a higher bandwidth. Can go upto 1MBit/S

· The fourth generation of mobile phones is 4G. 4G is still conceptual, but the enabling technology would be the removal of all circuit switch communication into a fully IP based packet switched integrated system. It will also combine with Wifi and Wimax infrastructure. Can go up to 100Mbit/S

  • LOS issue can be as follows

· Size :- Obstruction which effects the signal must e larger than then the wave length

· Material :- If an obstruction is a good conductor, it will reflect the signal which is not good. However if the obstruction is a good insulator, the signal will pass thorugh. The degree to which a material is a good conductor or insulator also depends on the wavelength. The higher the wave length, the better the signal may pass through the object

  • The current most popular technology for wireless transmission is the Motorola canopy
  • The general frequency are as follows

· 0 – 4KHz : Telephone audio inside cables

· 25KHz – 1MHz : Broadband Internet

  • Comparison between 900Mhz and 2.4GHz

· 900Mhz more range

· 900 Mhz has more interference from pager and mobile communication

· 900 Mhz can reach without LOS

· 900 Mhz has lower power consumption

  • Range of wireless transmission is limited by the following factors

· The type of signal in use (i.e. the underlying technology), similarly to the fact that AM radio waves reach further than FM radio waves

· The transmitter's rated power

Wireless Transmitters

  • Wireless transmitter work in the ISM band of 902-928Mhz. Development is currently being done for 2.4GHz which has less interference and higher band width
  • Data rate is 4.8 kbps, 19.2 kbps and 76.8 kbps
  • Has CRC check to ensure data accuracy
  • Wiring cost saving of RM40 per meter
  • The waves sent are omni directional in comparison with Motorola canopy which is angles at +45 vertical and +30 horizontal
  • Range is about 600m but varies accordingly
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