Series Circuit VS Parallel Circuit
A series circuit connects loads one after another in a single current path, so the same current flows through every element. A parallel circuit connects loads across the same supply through multiple branches, so each branch sees approximately the same voltage. The typical AGL lighting circuit is series, fed by a Constant Current Regulator (CCR); conventional building lighting is typically parallel, fed by a constant-voltage supply.
At a Glance
| Feature | Series Circuit | Parallel Circuit |
|---|---|---|
| Current paths | One | Multiple |
| Current through each load | Same | Depends on each branch/load |
| Voltage across loads | Divided among loads | Same supply voltage across branches |
| Supply philosophy | AGL: constant current | Conventional: constant voltage |
| If one load opens | Basic series loop: interrupted. AGL series circuit: isolation transformers are designed to keep the loop intact | Other branches can normally continue |
| Adding loads | Changes total circuit voltage/impedance | Mainly increases total current |
| AGL lighting | Typical series architecture | Not typical for the AGL primary lighting circuit |
| Conventional building lighting | Less common | Very common |
What Is a Series Circuit & a Parallel Circuit?
Series Circuit
A series circuit has one continuous current path, with loads connected one after another. The same current flows through L1, L2 and L3 in turn — I → [L1] → [L2] → [L3] → I.
Main characteristics
- Single current path through all loads
- Same current at every point in the circuit
- Supply voltage is divided among the loads
- Typical architecture for AGL lighting circuits
Parallel Circuit
A parallel circuit has multiple branches, each connected across the same supply. Each load therefore sees approximately the same supply voltage.
Main characteristics
- Multiple independent current paths
- Same voltage across every branch
- Total current is the sum of the branch currents
- Typical architecture for conventional building lighting
The Biggest Difference
The easiest way to remember the difference between a series circuit and a parallel circuit:
Series Circuit
One current path → same current
Parallel Circuit
Multiple current paths → same voltage
Easy memory: Series → same current. Parallel → same voltage.
Series vs Parallel Architecture
Series — AGL Circuit
Fixtures are wired one after another via isolation transformers; the CCR regulates the single series current shared by the whole circuit.
Parallel — Conventional Circuit
Each lamp is connected directly across the supply as an independent branch, each seeing approximately the same supply voltage.
Series-Connected Runway & Approach Circuits
The AGL series architecture allows a CCR to regulate the lighting circuit current and thereby provide controlled light output across a large number of fixtures distributed along a long route — a runway edge row, a taxiway row, or an approach lighting run. Isolation transformers interface each individual fixture with the series circuit.
Simplified architecture: CCR → Series Circuit → Isolation Transformers → Individual Light Fixtures. The CCR controls the circuit current; the transformers interface the constant-current series circuit with each individual lighting unit.
How Conventional Lighting Is Typically Wired
A conventional lighting system is typically arranged with each lamp connected across the supply. If the supply is 230 V, for example, each branch is approximately across the 230 V supply, subject to system conditions. Switching or failure of one branch normally does not remove power from the other branches.
Current in a Series Circuit vs a Parallel Circuit
| Aspect | Series Circuit | Parallel Circuit |
|---|---|---|
| Current through elements | Same through every element | Splits between branches |
| Total current | Equal to the single circuit current | Sum of the branch currents |
| Resistive total | R_total = R1 + R2 + … | Determined by parallel combination of branches |
| Depends on | The single-path circuit as a whole | Each individual branch/load |
For a real AGL series circuit, the exact circuit behaviour also depends on the isolation transformers, lamps/LED fixtures, cable impedance and CCR characteristics.
Effect of an Open-Circuit Fault
| Condition | Series Circuit | Parallel Circuit |
|---|---|---|
| One load opens | A basic series string is interrupted, but an AGL series circuit's isolation transformers are specifically designed to maintain circuit continuity when a lamp/fixture fails open | Other branches can normally continue operating |
| Practical consequence | An open circuit in the series cable/loop itself (e.g. a cable break) can still interrupt the whole circuit, so monitoring and fault-detection remain important | Failure is usually isolated to that branch |
An individual lamp or fixture failing open on an AGL series circuit is not, by itself, expected to interrupt the loop — that continuity is exactly what the isolation transformer is there to preserve (per IEC 61823 and equivalent equipment standards). What can interrupt the whole series circuit is a fault in the series cable or transformer primary itself, which is why circuit monitoring still matters, just not for the reason a simple series-circuit diagram might suggest.
Effect of Adding an Additional Load
Series
Adding another load increases the total series impedance. In a conventional voltage-source series circuit this affects current and voltage distribution; in AGL, the CCR adjusts/maintains the selected series current, while the required circuit voltage depends on the total circuit characteristics.
Parallel
Adding another branch increases the total current demanded from the constant-voltage supply. Each existing branch continues to see approximately the same supply voltage.
How Voltage Is Distributed
Series
Supply voltage divides among the series elements (V1, V2, V3…)
Parallel
Branches share the same supply voltage (V1 = V2 = V3)
Full Engineering Comparison
| Aspect | AGL Series Circuit | Conventional Parallel Circuit |
|---|---|---|
| Primary supply | CCR | Constant-voltage supply |
| Circuit arrangement | Series | Parallel |
| Controlled quantity | Current | Voltage |
| Individual light | Isolation transformer | Usually direct branch connection |
| Current | Same series current | Branch-dependent |
| Voltage | Circuit voltage distributed according to circuit characteristics | Same nominal supply voltage across branches |
| Open-circuit fault | A lamp/fixture failing open is normally isolated by the transformer; a cable/loop fault can interrupt the whole series circuit | Usually affects only that branch |
| Typical application | Runway/taxiway/approach lighting | Building/general lighting |
Where Each Architecture Wins
Advantages of Series
- Same current guaranteed through every fixture on the circuit
- Well suited to a CCR regulating a large number of distributed loads
- Consistent light output regardless of circuit length, when correctly regulated
- Isolation transformers keep an individual lamp/fixture failure from interrupting the rest of the circuit
- Standard architecture for long, distributed AGL circuits
Advantages of Parallel
- Each load receives the system voltage independently
- One branch failing normally does not affect the others
- Simple to add or remove independent branches
- Very familiar arrangement for general LV lighting
Series Circuit vs a Simple Chain of Switches
A series lighting circuit is not just loads wired end to end. On AGL circuits, isolation transformers interface each fixture with the series circuit, and a CCR actively regulates the circuit current — the arrangement is a regulated current loop, not simply a chain of unregulated loads.
Practical Difference
Think: Series
I = same everywhere → one path
Think: Parallel
V = same everywhere → many paths
Myths vs Facts
The isolation transformer arrangement, combined with circuit monitoring, is intended to keep the series circuit intact even when an individual fixture develops a fault — an open circuit is still a risk the design guards against, not an inevitable outcome.
Adding branches still increases the total current drawn from the supply, and supply/cable sizing must account for this — parallel branches are independent in operation but not entirely independent in their effect on the source.
Choosing Between Series and Parallel
When selecting or interpreting a lighting circuit's topology, consider:
- Whether the application requires guaranteed uniform current (series) or independent branch operation (parallel)
- Total circuit length and number of light positions
- Applicable ICAO/national AGL requirements for the primary lighting circuit
- Isolation transformer ratings and losses, for series AGL circuits
- Cable resistance and voltage drop over circuit length
- Consequence of a single load/branch failure for the application
- Protection, control and monitoring requirements
- Airport-specific technical specifications
Which Architecture Applies Where?
A series circuit gives every connected load the same current and is the typical architecture for AGL lighting, fed by a CCR through isolation transformers. A parallel circuit gives every branch the same supply voltage and is the typical architecture for conventional building lighting, fed by a constant-voltage supply.
The final selection for an actual airport project should always be based on the applicable current ICAO/national AGL standard, the approved circuit design and equipment requirements — not on a generic comparison alone.
Engineering & Expert Notes
Series Circuits Can Be Hazardous When Open
SafetyTreat any series circuit as live until isolated and proved dead.
Draw the Circuit Before Fault Finding
MethodKnow which topology you are on.
Interpret Voltage and Current in the Right Context
DiagnosisBehaviour differs from parallel circuits.
Teach New Staff the Difference First
TrainingIt underpins safe practice.
Frequently Asked Questions
In a series circuit, one current path means the same current flows through every load. In a parallel circuit, multiple branches share the same supply voltage while the total current splits between them.
A series arrangement allows a single CCR to regulate the current through a large number of distributed fixtures, giving controlled and consistent light output along a long circuit such as a runway edge row.
Normally, nothing beyond that fixture: the isolation transformer is specifically designed to keep the series loop intact when a lamp or fixture fails open, so the rest of the circuit stays lit. What can interrupt the whole series circuit is a fault in the cable or loop itself, not an individual lamp failure — that's the main risk circuit monitoring and fault-detection are there to catch.
The other parallel branches can normally continue operating, since each branch is an independent current path across the supply.
Parallel distribution is convenient because each load receives the system voltage independently, and switching or failure of one branch normally does not remove power from the others.
The CCR regulates the series-circuit current, while individual isolation transformers interface that circuit with each individual lighting unit, allowing the series current regulation to be maintained fixture by fixture.
Adding a series load changes the total series impedance, and in AGL the CCR adjusts to maintain the selected current. Adding a parallel branch mainly increases the total current demanded from the supply.
Governing Documents
For an actual airport project, verify the current applicable editions of the relevant standards and the specific CCR/equipment datasheets before using this comparison for project design.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — series circuit requirements, CCR characteristics, intensity steps and light-circuit configuration for AGL.
ICAO Aerodrome Design Manual
Applicable material covering series circuit design, CCR ratings, isolation transformer requirements and electrical infrastructure guidance.
FAA AC 150/5345-10
Specification for Constant Current Regulators and Regulator Monitors — the governing equipment spec for the CCR that drives the AGL series circuit.
National CAA / DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Exact circuit topology, CCR rating and monitoring/protection requirements can vary with the governing standard, equipment manufacturer and approved airport design. For final design, use the current applicable standard and manufacturer datasheets rather than a generic comparison page.
A series circuit gives the same current through every connected load and is the typical AGL lighting architecture; a parallel circuit gives the same voltage across every branch and is the typical architecture for conventional lighting.
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