INSTRUMENT CABLES

INTRODUCTION:

Instrument cables are special cables meant for use in signal transmission from sensing element to the final electronics or data acquiring system. The type, construction features, conductor size, ratings, etc. vary depending on industry application.  

INSTRUMENT CABLE TYPES:

The instrument cable types generally in use are signal cables, triad cables, T/C cable, Communication cable and control & power cable.

CONSTRUCTION STANDARDS:

 The following construction standards for cable are generally used. Other equivalent standards also may be used.

CABLE	STANDARD
Signal & triad cable	BS 5308 part 1&2
Control and power cable	IS 1554 part 1
Thermocouple compensating cable	IS 8784
T/C extension cable	ANSI 96.1, BS 4937, ISA MC 96.1

SIGNAL AND TRIED CABLES:

Construction wise both of these cables are same except in signal cable 2 –wires are twisted together to form a pair while 3 –wires are twisted to form a triad.

Following are the different parts of these types of cables:

• Conductor
• Insulation 
• Binder Tape 
• Individual pair screen and drain wire 
• Binder tape 
• Overall binder 
• Collective screen & drain wire 
• Binder tape
• Inner sheath
• Armour
• Outer sheath
• Communication pair (Optional)

Pair Identification:

As per BS: 5308 colour coding or numbered tape provided with each screened pair / triad cable. Alternatively numbers are printed on the primary insulation itself.

Look at the number and colour coding table as per the standard BS 5308 PART-2

Laying of Pairs:

Outer layer will be right hand lay and subsequent layers will be in opposite direction

All dimensions (i.e. diameter, width , and thickness ) are as per the standard BS : 5308

BS: 5308 PART-1 = Polyethylene insulated & sheathed cable.

BS: 5308 Part-2 = PVC insulated & sheathed cable.

CONTROL AND POWER CABLE:

Control cables are manufactured as per IS 1554 PART –1.

In control & power cable there are no screen and drain wire. Binder tape is also an option. Following are the different parts of control cables.

• Conductor
• Insulation
• Binder (optional)
• Inner sheath
• Armour
• Outer sheath

CORE IDENTIFICATION:

Upto 5 core: Red, yellow, Blue, Black & Gray

6 –core & above:

Two adjacent cores in each layer (counting & direction) will be blue and yellow and the rest will be gray or some colour and number printed on the insulation at every 50mm length.

CABLE CODE: (For power cable only)

       Constituent                                         Code letter

• Aluminium conductor                        A 
• PVC insulation                                   Y
• Steel wire armour                               W
• Steel double wire armour                  WW
• Steel strip armour                                F
• Steel double strip armour                   FF
• PVC outer sheath                                Y

Note: For copper conductor there is no code letter.

T/C COMPENSATING CABLE:

Compensating cables are manufactured as per IS 8784 standard.

A pair of wires having such emf temperature characteristics to the T/C with which the wires are intended to be used., that when properly connected to T/C , the effected reference junction is in effect transferred to the other end of the wires.

There are 13 types of twin-core compensating cables i.e. from Type 1 to Type 13.

For multicore compensating cable, it is Type 14. Again depending on the number of pairs, there are 7 categories of multicore cables i.e.

Type 14A        :           2 pairs

Type 14B        :           4 pairs

Type 14C        :           6 pairs

Type 14D        :           8 pairs

Type 14E         :           10 pairs

Type 14F         :           12 pairs

Type 14G        :           36 pairs

Recommended materials: T/C comp. Cable:

Type	+lead	-lead	Thermo-emf
Cu-Con Black	Copper Blue	Constantan or Equiv. Red	IS: 2056
Iron-Constantan Blue	Iron Blue	Constantan or Equiv. Red	IS:2057
Chromel- Alumel Green	Iron / Copper Green	Copper –Nickel Alloy Red	IS:2054
Pt-Rh-Pt White	Copper White	Copper –Nickel Alloy Red	IS:2055
Pt /Rh – Pt/Rh Yellow	Copper Yellow	Copper –Nickel alloy Red	IS: 6720

Following are the different parts of T/C comp. Cable:

Conductor size: minimum 1mm of diameter.

• Insulation:

            Rubber         (IS: 8380)

            PVC     (IS: 5831)

                 Glass  fibre

•  Shielding (plain or tined braided copper / Heat resistant aluminium foil):
•    Inner sheath           
•  Armour           
•  Overall Sheath

Colour Coding: As per Table – 5 of IS : 8784.

T/C EXTENSION CABLE:

The following standards are to be followed.

ANSI MC 96.1 OR ISA MC 96.1

BS: 4937 –30

Different types of T/C are designated as follows:

Type	Material		Range
	Positive	Negative
B	Pt-30% Rh.	Pt-6% Rh	0 – 1820 °C
E	Ni-10 %Cr	CuNi	-270 – 1000 °C
J	Fe	CuNi	-270 – 760 °C
K	Chromel	Alumel	-270 – 1372 °C
T	Cu	Constantan	-270 – 400 °C.

Symbols for types of extension cable

                        Type             Combination                Positive                           Negative

                        B                     BX*                            BPX                                 BNX

  

                        E                      EX                               EPX                               ENX

                        J                       JX                                JPX                             JNX                                                                

                        K                     KX                              KPX                                KNX

                        R&S                SX*                             SPX                                SNX

                        T                      TX                               TPX                               TNX

             * =Compensating cable.

Colour code (Extension cable) As per ISA MC 96.1.

                        Type              Overall                        Positive                   Negative

                       

                        B                     Gray                            Gray                            Red

                        E                      Purple                         Purple                          Red

                        K                     Yellow                        Yellow                         Red

                        J                       Black                          White                          Red

                        R&S                Green                          Black                           Red

                        T                      Blue                            Blue                             Red

CABLE MATERIAL:

CONDUCTOR:

Annealed copper conductor as per IS: 8130 or BS: 6360

Dimension, resistivity and other mechanical properties as per the above  standards

Tin coating on annealed copper conductor is specified whenever the atmosphere is

corrosive for bare copper.

Generally multi-strand (flexibility class –2) copper conductor are used.

Drain wire 0f 0.5 mm2 tinned annealed copper conductor is used.

Minimum 10 twists per meter are required for pair / triad cable.

Refer BS: 6360 table.

INSULATION:

Generally two types of insulation materials are specified

a.                   PVC (Poly vinyl Chloride)  as per IS : 5831 or BS : 6746

b.                  Poly-ethylene as per IS : 7098 Part –1

For further details refer standards listed above.

PVC material as per IS: 5831 are having the following grades:

                        Type A: Max. : Max. conductor temp. allowed is 70 °C with rated voltage upto and including 3.3. kV.

                         Type B: Same as above except voltage is above 3.3. kV

                         Type C: Heat resistant up to conductor temperature. 85°C with rated voltage upto and including 1100 Volts.

                         Type ST1: Used for sheath upto conductor. 70 °C.

                         Type ST2: Heat Resistant sheath , used upto conductor. 90 °C.

Types A, B & C are generally used for primary insulation and Type ST1 & ST2 are for sheathing.

For FRLS (Flame retardant low smoke) cable “ Aluminium trihydrate” is mixed with PVC compound to make the cable flame retardant.

Polyethylene cable can never be flame retardant.

Polyethylene cable has better insulating properties and more resistant to ingress of moisture. But the colour of cable gets faded under sunlight and rain within a year. Manufacturers mix anti UV compound with carbon black polyethylene compound to improve colour fastness but cannot give guarantee of colour.

Polyethylene armoured cables are suitable for underground laying.

Thickness of insulation will be as per the respective cable standard but other properties will be as per IS: 5183 for PVC and IS: 7098 part for polyethylene.

BINDER TAPE

It should be non-hygroscopic material, generally polyester tape.

 SCREEN

Aluminium bonded laminated polyester tape, of minimum thickness of a minimum 0.008-mm and polyester tape 0.01 mm continuously in contact with drain wire. Screen tape shall be applied with a minimum overlap of 25 %. Braided type of screen is provided to restrict low freq. Interference. Iron tape or MU metal is used for magnetic interference.

SHEATH (INNER &OUTER):

Depending on the type of cable either PVC or POLYETHYLENE is used.

Generally most of the instrument Cables are PVC sheathed cable.

PVC type ST1 for normal application and for heat resistant PVC type ST2 should be specified.

ARMOUR

Armouring is provided for instrument cables as per IS3975 or BS:1442. The armouring either consists of steel wire or aluminium flats as per following table.

Cable diameter over “Inner Sheath”	Armouring material
Less than or equal to 13 mm	Galvanised steel wire of 0-9 mm diameter
Greater than to 13 mm	Galvaniser iron strip of 4 x 0.8 mm

CABLE TESTING:

There are different types of testing done on cables before use.

TESTS AND STANDARDS:

TESTS	STANDARDS
1. Conductor resistance test	IS: 8130 & BS : 6360
2. Annealing test of copper	IS: 8130 & IS : 10810 PART –1
3.Tensile strength & elongation test	IS : 5831 & IS : 10810 Part – 7
4. Insulation resistance test	IS: 5831 & IS : 10810 Part –43
5. High voltage test	BS: 5308 / IS : 1554 / IS : 10810 –45
6. Flame retardant test	IS : 10810 PART –61
7. Oxygen index test	ASTM D 2863 & IEC 332 –3
8. Smoke density test	ASTM D 2843 & IS : 10810 PART –63
9. Halogen Acid gas test	IEC –754 –1
10. Capacitance , Inductance , & L/R ratio test	BS: 5308 Part –2 or Part -1

FOR DETAILED DESCRIPTION ABOUT CABLES REFER SATANDARDS WHICH ARE USED IN THIS BLOG AND PDFs RELATED WITH THEM.