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Tuesday, September 9, 2008

Instrument Power Cable Sizing

Sizingofinstrument cableshould be performed to ensure that instrument device such as

solenoid valveand its cabling works properly. There are two factors that should be checked

i.e. voltage drop and current carrying capacity.

Voltage drop iscalculatedas follows:

Vdrop = [Rio + (Rw x 2L) + Rins] x In

where:

Rio = Input resistance of I/O Card (Ohm)

Rw = Resistance ofwireat specified temperature per lenght (Ohm/m)

Rins = Resistance of instrument device (Ohm)

L = Length of cable / distance from instrument to cabinet in control room (m)

In = Load current (A), obtained from power consumption of instrument device divided by

power supply voltage.

Rins, Rio and Rw can be obtained from manufacturers catalog for instrument, I/O card and

cable respectively.

The cable suits the application if calculated voltage drop is less than maximum permissible

voltage drop by the system.

Current Carrying Capacity of a certain cable is defined as the maximum current that can

flow through a cable without melting the conductor or insulation. Current Carrying Capacity

varies depending on several factors such as conductor size, ambient temperature, and

cableinstallation. These factors are referred as derating factors and IEC has established

the values for each.The cable suits the application if nominal current (In) is less than current

carrying capacity.

Labels:bulk material,sizing

Posted by Instrumentcontrol at1:18 PM

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1 comments:

Anonymous said...

Excellent information . Thanks a lot.

February 8, 2010 at 7:08 PM

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Page 2 of 2Instrument and Control Engineering: Instrument Power Cable Sizing

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