Table of Contents
What Is Electrical Resistance?
Electrical resistance is one of the most fundamental quantities in electrical engineering - and it also plays a central role in real power networks.
Electrical resistance explained simply:
The greater the resistance, the more difficult it is for current to flow through a conductor or component.
Typical questions behind this are:
- Why does more or less current flow in a circuit at the same voltage?
- Why do cables and contacts heat up?
- Why does the voltage drop at the end of a long cable?
We only touch briefly on insulation resistance here - a detailed explanation is provided in a separate article on the measurement of insulation resistance.
Resistance - Definition in Physics and Electrical Engineering
Definition of Resistance in Physics
In physics, electrical resistance 
is defined as the ratio of voltage to current :
Definition:
The electrical resistance of a component is the quotient of the applied electrical voltage and the current I flowing through it.
- Symbol for resistance: R
- Voltage: U
- Current:
I
This definition of electrical resistance is the basis for Ohm's law and ultimately answers the question:
"What does resistance mean in a circuit?"
What Is a Resistor as a Component?
In electronics and electrical engineering, the term resistor often refers to the specific component:
- A resistor is a passive component with a defined electrical resistance value.
- It is used to limit currents, divide voltages, adapt signals or generate defined voltage drops for measurement and control purposes.
Typical applications:
- Series resistor for LEDs
- Voltage divider for signal adaptation
- Measuring resistor or shunt for current measurement
- Pull-up and pull-down resistors in digital circuits
Unit of Electrical Resistance
Unit of Resistance
The unit of electrical resistance is the ohm.
- Unit of resistance: ohm
- Unit symbol: Ω
- Notation: 1 Ω, 10 Ω, 4.7 kΩ, 1 MΩ ...
Common multiples:
- kΩ (kiloohm) = 1,000 Ω
- MΩ (megaohm) = 1,000,000 Ω
Current, Voltage and Resistance Explained Simply - Ohm's Law
Ohm's law describes the relationship between current, voltage and resistance:
This results in the common transformations:
- Calculating resistance using
– - Calculating current using
– - Calculating voltage using
Resistance explained simply:
- If you increase the voltage while resistance remains constant, the current increases.
- If you increase the resistance while voltage remains constant, the current decreases.
Example:
- Given:
, 
- Required: resistance
This makes it easy to calculate resistance in a circuit from measured voltage and current.
Resistance in a Circuit - Practical Classification
Circuit with a Resistor
A simple circuit with a resistor consists of:
- a voltage source, such as a power supply unit or battery,
- a resistor as the load
- and the connecting cables.
In this circuit:
- the resistor limits the current,
- the voltage drops across the resistor,
- electrical power is converted into heat in the resistor:
reference is made to the fundamentals of resistance and its temperature dependence.
Resistance and Real Cables
In real electrical engineering, not only component resistors are relevant, but also the resistance of:
- cables and conductors made of copper or aluminium
- contacts, terminals and switches
- transition points, such as plug connections
Consequences:
- voltage drop on long cables
- losses in the form of heat, known as I²R losses
- limitation of short-circuit currents through line and network impedances
For grid planners and operating engineers, resistance in a circuit is therefore directly relevant for:
- dimensioning cable cross-sections
- estimating power quality
- designing and coordinating the selectivity of protection devices
Evaluating Network Impedance and Resistance
Mobile measurement solutions for analysing voltage drop, short-circuit power and network stability
Electrical Resistance in the Power Supply Network
In electrical power supply, the term resistance appears at several levels.
Line Resistance and Network Impedance
- Every cable has a resistance, determined by material, length and cross-section.
- In AC networks, impedance is often considered, meaning the combination of resistance and reactive component.
- In this context, resistance is the active resistance, or ohmic component, of impedance.
Effects:
- Greater line resistance → higher voltage drop under load
- Greater line resistance → lower short-circuit currents
- Network impedance influences power quality and immunity to disturbances, for example in relation to harmonics.
Load Resistance
In the simplest model, every load in the network can be described by a load resistance:
- High load resistance → low current
- Low load resistance → high current
Many network situations can be roughly modelled with "source - cable - load resistance". This simple model is often sufficient to estimate:
- voltage drops
- load flows
- thermal stresses on equipment.
Insulation Resistance
In addition to line resistance and load resistance, insulation resistance plays an important role in electrical installations:
- It describes how well active conductors are insulated against earth or against each other.
- A high insulation resistance means low leakage currents and contributes to operational safety.
Overview of Resistance Measurement
Measuring Resistance - Classical Resistance Measurement
Various methods are available for resistance measurement:
- Direct measurement with a multimeter or ohmmeter
- Low test voltage
- Suitable for components, short cables and contact resistance
- Four-wire measurement, also known as Kelvin measurement
- For very low resistance values, for example shunts or busbars
- Separately routed current and voltage circuits minimise measurement errors caused by cable and contact resistance
Typical applications in network and plant operation:
- Measurement of contact resistance at terminals or connections
- Testing of earthing systems, where the term earthing resistance is used - the principle is related, but application-specific.
From component measurement to network assessment
Classical ohmmeters are suitable for components and short cables. For networks and installations, however, continuous voltage and current measurements are required. Power quality and monitoring devices from A. Eberle continuously record operating voltages, currents and other key parameters, supporting the assessment of losses, voltage drops and critical operating states.
Learn more
Insulation Resistance Measurement
The measurement of insulation resistance differs from normal resistance measurement mainly through:
- significantly higher test voltages
- target values in the MΩ to GΩ range
- safety-related requirements for preparation and execution.
More Theory: Material Dependence of Resistance
For many planning and calculation tasks, the material- and geometry-dependent description is important:
with
: specific resistance of the material
: length of the conductor
: cross-sectional area
This immediately makes clear:
- Longer cable → greater resistance
- Larger cross-section → lower resistance
- The choice of material, such as copper versus aluminium, directly affects resistance.
For engineers in electrical engineering and power supply, this relationship is central for:
- dimensioning cable cross-sections
- calculating voltage drop and losses
- assessing heating and thermal load limits of equipment.
Resistance, Impedance and Other Quantities - Classification
In practical network operation, engineers rarely work only with "the" resistance. The following quantities are often decisive:
- Resistance (R) - active component
- Reactance (X) - reactive component, for example coils or capacitors
- Impedance (Z) - combination of R and X
Nevertheless, electrical resistance remains the fundamental quantity:
- It determines the active effects, such as losses and heating
- It is included in every impedance calculation
- It is essential for short-circuit current calculations, protection coordination and network analyses.
Typical Questions from Practice - FAQ
1. What is the exact definition of resistance in physics?
Resistance is the quotient of applied voltage and flowing current (R=U/I). It describes how strongly a component or conductor opposes current flow.
2. What is the difference between "resistance" and "resistance in electronics"?
• Resistance as a physical quantity: a property of every conductor or component.
• Resistance in electronics: a specific component, also called a resistor, with a defined resistance value that is used deliberately in circuits.
3. How do I measure resistance correctly?
• For "normal" resistance values: with a multimeter in the ohm range, with the circuit disconnected from voltage.
• For very low or very high resistance values: with specialised methods such as four-wire technology, bridge circuits or suitable measuring devices.
4. What role does resistance play specifically in the power network?
• Together with the network structure, it determines short-circuit currents.
• It influences voltage drop and line losses.
• It is important in fault analysis, for example increased contact resistance or heated contacts.
5. Where does insulation resistance fit in?
Insulation resistance describes the resistance values of the insulation between active parts and earth or between conductors. It is a measure of insulation quality and is primarily considered during testing and diagnostics.
"Insulation resistance in electrical networks".
Conclusion
Electrical resistance is a central quantity in electrical engineering and power engineering:
- It links voltage and current through Ohm's law.
- It describes the behaviour of components, cables and contacts - from electronics to high-voltage networks.
- It is a key quantity for network calculation, protection technology, loss analysis and the assessment of real network states.
Topics such as insulation resistance and its measurement build on this basic understanding and are classified in a separate, more in-depth article.
If you would like to examine this topic more closely in the context of your specific network situation - for example for short-circuit current calculations, voltage drop analyses or the assessment of line losses - we will be happy to help you derive the right procedure and suitable measurement technology.
