Introduction to Basic Electrical Fundamentals

Instrumentation and control systems depend on electrical principles. Field instruments, transmitters, switches, control valves, PLC input/output modules, relays, solenoids, indicators, and control panels all use electrical signals or electrical power in one form or another.

An instrumentation technician must understand basic electrical quantities, AC and DC supply, Ohm’s Law, Kirchhoff’s Laws, series and parallel circuits, electrical symbols, and circuit diagrams. This knowledge helps in wiring, testing, calibration, loop checking, troubleshooting, and safe maintenance.

Electrical work must always be approached with caution. OSHA provides electrical safety standards because electrical hazards can cause shock, burns, fire, arc flash, equipment damage, and death.

Electrical Quantities

Electrical quantities are the basic values used to describe how electricity behaves in a circuit. The most important quantities for instrumentation work are voltage, current, resistance, and power.

Quantity	Symbol	Unit	Meaning
Voltage	V	Volt	Electrical pressure that pushes current through a circuit
Current	I	Ampere	Flow of electric charge
Resistance	R	Ohm	Opposition to current flow
Power	P	Watt	Rate at which electrical energy is used or converted

The SI unit of electric current is the ampere, while common derived electrical units include the volt, ohm, and watt. NIST defines the volt as 1 W/A and the ohm as 1 V/A.

Voltage

Voltage is the electrical potential difference between two points. It is the force that pushes current through a circuit.

In instrumentation, voltage may appear as:

• 24 V DC control supply
• 110 V AC control supply
• 230 V AC power supply
• 0–10 V analog signal
• Millivolt signal from thermocouples
• Low-voltage signals in electronic circuits

Voltage must be measured between two points. For example, when checking a 24 V DC supply, the meter probes are usually placed across the positive and negative terminals.

Current

Current is the flow of electric charge through a conductor. It is measured in amperes.

In instrumentation, current is very important because many industrial transmitters use a 4–20 mA signal. In this signal range, 4 mA usually represents the lower measurement value, while 20 mA represents the upper measurement value.

Examples:

Signal	Possible Meaning
4 mA	0% process value
12 mA	50% process value
20 mA	100% process value

Current must be measured in series with the circuit, unless a clamp meter or loop calibrator method is being used. Incorrect current measurement can damage the meter or circuit.

Resistance

Resistance is the opposition to current flow. It is measured in ohms.

Resistance is found in:

• Cables
• Resistors
• RTD temperature sensors
• Coils
• Relay windings
• Solenoids
• Poor connections
• Corroded terminals
• Damaged conductors

In instrumentation, resistance is important when testing RTDs, checking cable continuity, identifying open circuits, and diagnosing high-resistance connections.

A very high resistance may indicate an open circuit. A very low resistance may indicate a short circuit, depending on the circuit being tested.

Power

Electrical power is the rate at which electrical energy is used or converted into another form such as heat, light, sound, magnetic force, or mechanical movement.

Power is measured in watts.

In instrumentation and control systems, power is used by:

• Transmitters
• Controllers
• PLC modules
• Relays
• Solenoid valves
• Indicators
• Control panels
• Power supplies
• Actuators
• Signal converters

The basic power formula is:

P = V × I

Where:

• P = Power in watts
• V = Voltage in volts
• I = Current in amperes

Example:

If a device operates at 24 V DC and draws 0.5 A:

P = 24 × 0.5 = 12 W

The device consumes 12 watts.

AC and DC Fundamentals

Electrical supply can be AC or DC.

Type	Full Meaning	Description
DC	Direct Current	Current flows in one direction
AC	Alternating Current	Current changes direction repeatedly

Both AC and DC are used in instrumentation and control systems.

Direct Current: DC

Direct current flows in one direction. DC is common in control circuits and instrumentation signals.

DC is used in:

• 24 V DC instrument power supplies
• 4–20 mA signal loops
• 0–10 V signals
• PLC input circuits
• Electronic instruments
• Battery systems
• Solar and backup systems
• Sensor circuits

In DC circuits, polarity matters. The positive and negative terminals must be connected correctly. Reversed polarity can stop a device from working or damage electronic equipment.

Alternating Current: AC

Alternating current changes direction repeatedly. AC is commonly used for power supply in buildings and industries.

AC is used in:

• Motor circuits
• Control transformers
• Lighting circuits
• Panel power supply
• Some solenoid valves
• Pumps and fans
• Heaters
• Industrial equipment

AC supply is described by voltage and frequency. Frequency is measured in hertz. In many countries, AC frequency is either 50 Hz or 60 Hz.

AC vs DC in Instrumentation

Feature	AC	DC
Direction	Changes direction repeatedly	Flows in one direction
Common use	Power supply, motors, heaters	Instruments, signals, PLC circuits
Polarity	Usually live and neutral/phase conductors	Positive and negative terminals
Measurement	AC voltage/current meter setting	DC voltage/current meter setting
Instrument example	230 V AC panel supply	24 V DC transmitter supply

A technician must select the correct meter function before testing. Measuring DC using the AC setting, or AC using the DC setting, can give incorrect readings.

Ohm’s Law

Ohm’s Law explains the relationship between voltage, current, and resistance.

V = I × R

Where:

• V = Voltage in volts
• I = Current in amperes
• R = Resistance in ohms

The formula can be rearranged:

Formula	Use
V = I × R	To find voltage
I = V ÷ R	To find current
R = V ÷ I	To find resistance

OSHA training materials include Ohm’s Law as an important part of electrical theory and electrical safety understanding.

Ohm’s Law Example

A 24 V DC circuit has a resistance of 12 Ω.

To find the current:

I = V ÷ R

I = 24 ÷ 12

I = 2 A

So, the current flowing in the circuit is 2 amperes.

Ohm’s Law is useful when checking voltage drops, estimating current, understanding resistor circuits, and diagnosing faults in basic control circuits.

Ohm’s Law in Instrumentation Work

Instrumentation technicians use Ohm’s Law when:

• Checking voltage drop in long cable runs
• Testing resistance in RTD sensors
• Understanding current loops
• Checking power supply loading
• Diagnosing open and short circuits
• Calculating resistor values
• Understanding relay and solenoid circuits

Example:

If a 250 Ω resistor is used in a 4–20 mA loop, the voltage across the resistor changes with current.

At 4 mA:

V = 0.004 × 250 = 1 V

At 20 mA:

V = 0.020 × 250 = 5 V

This is why a 250 Ω resistor can convert a 4–20 mA signal into a 1–5 V signal for some control systems.

Kirchhoff’s Laws

Kirchhoff’s Laws are used to analyse electrical circuits. They are especially useful when circuits have multiple branches, loops, or components.

There are two main laws:

Law	Meaning
Kirchhoff’s Current Law	Current entering a junction equals current leaving the junction
Kirchhoff’s Voltage Law	Total voltage around a closed loop equals zero

Kirchhoff’s Current Law is useful for analysing current at nodes or junctions, while Kirchhoff’s Voltage Law is useful for analysing voltage drops around loops.

Kirchhoff’s Current Law: KCL

Kirchhoff’s Current Law states that the total current entering a junction is equal to the total current leaving the junction.

In simple terms:

Current in = Current out

Example:

If 2 A enters a junction and splits into two paths, and one path carries 0.8 A, the second path must carry:

2 A − 0.8 A = 1.2 A

KCL is useful when checking parallel circuits and current distribution.

Kirchhoff’s Voltage Law: KVL

Kirchhoff’s Voltage Law states that the total voltage supplied in a closed circuit loop is equal to the total voltage drops around that loop.

In simple terms:

Supply voltage = Sum of voltage drops

Example:

A 24 V DC circuit has two loads in series. If one load drops 10 V, the other load must drop:

24 V − 10 V = 14 V

KVL is useful when checking voltage drops across components in control circuits.

Series Circuits

A series circuit has components connected one after another in a single current path.

In a series circuit:

• Current is the same through all components.
• Voltage is shared across the components.
• Total resistance increases when more resistance is added.
• If one component opens, the whole circuit stops working.

Example:

A fuse, switch, and lamp connected one after another form a simple series circuit. If the switch is open, current cannot flow and the lamp will not turn on.

Total Resistance in Series

For resistors in series:

R total = R1 + R2 + R3

Example:

If three resistors are connected in series:

• R1 = 10 Ω
• R2 = 20 Ω
• R3 = 30 Ω

Total resistance:

10 + 20 + 30 = 60 Ω

The total resistance is 60 Ω.

Series Circuit in Instrumentation

Series circuits are found in:

• Fuse and device supply paths
• Switch circuits
• Interlock circuits
• Emergency stop chains
• Some relay control circuits
• 4–20 mA loops

A 4–20 mA loop is normally wired in series so that the same loop current flows through the transmitter, power supply, receiving device, and loop resistor where used.

Parallel Circuits

A parallel circuit has more than one path for current to flow.

In a parallel circuit:

• Voltage is the same across each branch.
• Current divides between branches.
• Total current is the sum of branch currents.
• If one branch opens, other branches may continue working.

Parallel circuits are common in control panels because several devices may need the same supply voltage.

Total Resistance in Parallel

For resistors in parallel:

1/R total = 1/R1 + 1/R2 + 1/R3

Example:

If two resistors are connected in parallel:

• R1 = 10 Ω
• R2 = 10 Ω

Then:

1/R total = 1/10 + 1/10

1/R total = 2/10

R total = 5 Ω

The total resistance is 5 Ω.

In parallel circuits, total resistance is always lower than the smallest branch resistance.

Parallel Circuit in Instrumentation

Parallel circuits are found in:

• Panel power distribution
• PLC input supply circuits
• Indicator lamps
• Multiple devices sharing 24 V DC supply
• Alarm circuits
• Control panel auxiliaries
• Some sensor power circuits

When troubleshooting parallel circuits, one device may fail while others continue working. This is different from a series circuit, where one open component can stop the entire path.

Series vs Parallel Circuits

Feature	Series Circuit	Parallel Circuit
Current	Same through all components	Divides between branches
Voltage	Shared across components	Same across each branch
Resistance	Adds together	Total is lower than smallest branch
Fault effect	One open fault stops the full path	One branch fault may not stop other branches
Instrument example	4–20 mA loop	Multiple devices on one 24 V DC supply

Understanding the difference helps technicians locate faults faster.

Electrical Symbols

Electrical symbols are graphic signs used to represent components in drawings and diagrams. They allow circuits to be shown clearly without drawing the actual physical device.

Common electrical symbols include:

Symbol / Component	Meaning
Battery / DC supply	DC power source
AC supply	Alternating current source
Resistor	Limits current or creates voltage drop
Switch	Opens or closes a circuit
Fuse	Protects circuit by melting during excess current
Circuit breaker	Trips during overload or fault
Lamp / indicator	Shows circuit status
Relay coil	Electromagnetic control device
Relay contact	Opens or closes by relay action
Ground / earth	Safety or reference connection
Motor	Converts electrical energy to mechanical rotation
Solenoid	Electromagnetic actuator
Transformer	Changes AC voltage level
Terminal	Connection point
Junction	Point where conductors connect

Symbols may vary by standard, company, or drawing style. Always check the drawing legend.

Instrumentation Symbols

Instrumentation drawings also use symbols and tag letters to represent instruments and control functions.

Examples:

Tag	Meaning
PT	Pressure Transmitter
TT	Temperature Transmitter
FT	Flow Transmitter
LT	Level Transmitter
PI	Pressure Indicator
TIC	Temperature Indicating Controller
XV	On/off valve
CV	Control valve
LS	Level Switch
PS	Pressure Switch

ISA-5.1 is a widely used standard for instrumentation symbols and identification. ISA standards support consistent automation and instrumentation documentation across industries.

Circuit Diagrams

A circuit diagram shows how electrical components are connected. It helps technicians understand how a circuit works, where power comes from, how signals travel, and where faults may occur.

Circuit diagrams are used for:

• Control panels
• Instrument loops
• PLC input/output circuits
• Relay circuits
• Power supply circuits
• Alarm circuits
• Motor control circuits
• Solenoid valve circuits

A technician should read the diagram before testing or disconnecting wires.

Types of Circuit and Instrument Diagrams

Diagram Type	Purpose
Schematic diagram	Shows how a circuit works logically
Wiring diagram	Shows physical wire connections
Loop diagram	Shows one complete instrument signal loop
P&ID	Shows process equipment, instruments, and control functions
Ladder diagram	Shows relay or PLC logic in ladder format
Panel layout drawing	Shows physical placement of panel components
Terminal diagram	Shows cable and wire termination points

Each drawing has a different purpose. A loop diagram is not the same as a panel layout, and a P&ID is not the same as a wiring diagram.

Reading a Simple Instrument Loop Diagram

A basic 4–20 mA loop diagram may show:

• 24 V DC power supply
• Transmitter
• Junction box
• Field cable
• Control panel terminal
• PLC or DCS analog input
• Loop resistor where required
• Cable number
• Wire colour
• Terminal numbers
• Earth/shield connection

A technician should trace the signal from the power supply to the field instrument and back to the receiving device.

Safe Use of Test Instruments

Electrical fundamentals are useful only when applied safely. Before testing, the technician must use the correct meter, select the correct range, and understand whether the circuit is live or isolated.

Safe testing practices include:

• Inspect the meter and test leads.
• Select AC or DC correctly.
• Select voltage, current, resistance, or continuity correctly.
• Do not measure resistance on a live circuit.
• Keep fingers behind probe guards.
• Do not short terminals with probe tips.
• Use meters with the correct safety rating.
• Follow permit and isolation requirements.
• Record readings accurately.
• Ask for guidance if unsure.

A common beginner mistake is placing the meter leads in the current socket and then trying to measure voltage. This can blow the meter fuse or create a short circuit.

Common Electrical Faults in Instrumentation

Fault	Possible Cause
No transmitter signal	No power supply, open loop, wrong wiring, faulty transmitter
Signal fixed at 0 mA	Open circuit, no power, disconnected wire
Signal below 4 mA	Loop fault, transmitter fault, power issue
Signal above 20 mA	Overrange process value, wiring fault, transmitter fault
Unstable signal	Loose terminal, poor shield grounding, electrical noise
PLC input not responding	Wrong terminal, failed input module, no signal
Solenoid not energising	No voltage, faulty coil, open control contact
Fuse keeps blowing	Short circuit, wrong fuse rating, faulty device
Control relay not operating	No coil voltage, bad contact, faulty relay

Basic electrical knowledge helps technicians diagnose these faults logically instead of guessing.

Real-Life Scenario

A technician is asked to check why a pressure transmitter is not showing a reading in the control room. The technician sees that the transmitter display is off and immediately assumes the transmitter is faulty.

A better approach is to check the electrical basics first. Confirm the 24 V DC supply, check polarity, inspect the loop fuse, check cable continuity, verify terminal tightness, and test the signal loop. The problem may be a loose wire or missing power supply, not the transmitter itself.

Good troubleshooting starts with basic electrical checks.

Common Beginner Mistakes

Avoid these mistakes:

• Confusing voltage and current.
• Measuring current the same way as voltage.
• Using AC meter setting for DC circuits.
• Testing resistance on a live circuit.
• Ignoring polarity in DC circuits.
• Assuming all control circuits use the same voltage.
• Mixing up series and parallel circuits.
• Misreading electrical symbols.
• Working from the wrong drawing.
• Shorting terminals with meter probes.
• Ignoring loose or corroded terminals.
• Replacing devices before checking power supply and wiring.

What an Instrumentation Technician Should Never Do

An instrumentation technician should never:

• Test live circuits without training and authorisation.
• Use damaged meters or test leads.
• Touch exposed terminals carelessly.
• Measure resistance on an energised circuit.
• Bypass fuses or protective devices.
• Guess wire connections without checking drawings.
• Disconnect instrument wires without labelling or recording.
• Ignore polarity on DC circuits.
• Use the wrong meter range or input socket.
• Leave panel doors or junction boxes open after work.
• Assume low-voltage circuits are always harmless.
• Hand over a circuit without testing and confirming operation.

Quick Recap

Basic electrical fundamentals are essential for instrumentation and control work. Voltage is electrical pressure, current is charge flow, resistance opposes current, and power is the rate of electrical energy use. DC flows in one direction, while AC changes direction repeatedly. Ohm’s Law relates voltage, current, and resistance. Kirchhoff’s Laws help analyse current at junctions and voltage around loops. Series circuits have one current path, while parallel circuits have multiple paths. Electrical symbols and circuit diagrams help technicians understand, install, test, and troubleshoot instrument circuits safely and accurately.