Electricity Distribution Diagram
This is a free printable electricity distribution diagram: download the diagram as SVG or open it and print to paper or PDF.
An electricity distribution diagram maps the path of electrical power from the transmission grid through substations and distribution networks to the consumer connection point, showing voltage transformation stages and protection at each level.
An electricity distribution diagram represents the infrastructure that steps down high transmission voltages and delivers usable electrical power to homes, businesses, and industrial facilities. Understanding this diagram is essential for electrical engineers, planners, and technicians working on grid integration, solar/battery installations, or any project that interfaces with the public network.
The distribution system begins where the high-voltage transmission grid ends — at the bulk supply point (BSP) or grid substation, where transmission voltages (typically 132 kV to 765 kV, depending on the region) are stepped down to sub-transmission voltages (typically 11 kV to 66 kV). This is the first transformation stage shown in the diagram.
From the grid substation, the sub-transmission network carries power to primary distribution substations (also called zone substations or primary switching stations). Here, a second transformer steps the voltage down again — typically to the medium-voltage (MV) distribution level of 6.6 kV, 11 kV, 22 kV, or 33 kV, depending on the regional network operator's standard.
The MV distribution network — the part most visible in urban and suburban areas as overhead lines or underground cables — carries power to secondary distribution substations (also called pad-mount transformers, kiosk substations, or pole-top transformers). At the secondary substation, the voltage is stepped down to the low-voltage (LV) supply level: 230/400 V in IEC countries (single-phase 230 V, three-phase 400 V), or 120/208 V or 120/240 V in North American systems.
The LV network then distributes power to individual consumer connection points via service cables.
At each level, the distribution diagram shows: - Transformers (with voltage ratio and kVA rating) - Circuit breakers or reclosers (protection and switching) - Fuses (LV protection at secondary substation and service level) - Earthing (system earth at each transformer neutral) - Metering points (at the connection point between network and consumer)
The diagram also indicates the system earthing arrangement — TN-S, TN-C-S, TT, or IT — which determines how the neutral and protective earth conductors are related throughout the network, and directly affects the design of consumer earthing installations.
How to wire electricity distribution diagram
- Identify the system boundary and scope Determine whether the diagram covers the full distribution chain from transmission to consumer connection, or only a portion (e.g., a single zone substation and its feeders). Define the highest and lowest voltages to be shown. This determines the level of detail required and the number of transformation stages to include.
- Draw the transmission/bulk supply connection Show the incoming transmission line voltage and the grid substation (or bulk supply point) at the top of the diagram. Indicate the transmission voltage, transformer rating (in MVA), and the sub-transmission output voltage. Include the main circuit breaker and transformer protection symbols.
- Show the sub-transmission and primary distribution substation Draw the sub-transmission network from the grid substation to the primary distribution substation (zone substation). Show the substation transformer, busbars, feeder circuit breakers, and protection relays. Label voltages on both sides of the transformer and the busbar arrangement (single busbar, double busbar, ring bus, etc.).
- Draw the medium-voltage (MV) distribution feeders Show the MV feeders leaving the primary substation. Indicate whether feeders are overhead or underground, their route (where relevant), and any mid-feeder switching points or reclosers. Show the ring or radial topology of the MV network. Label each feeder with its designation and protection settings if known.
- Show secondary substations and LV networks Draw each secondary substation (distribution transformer) connected to the MV feeder. Show the transformer kVA rating, MV voltage, and LV output voltage. Below the transformer, draw the LV busbars and outgoing LV feeders supplying consumer service connections. Show LV fuses or MCCBs at the substation output.
- Add system earthing and protection symbols At each transformer, show the earthing connection on the neutral point. Label the earthing arrangement (TN-S, TT, etc.). Show protection relay types at each level (overcurrent, earth fault, differential) using standard IEC 60617 symbols. Add metering points at the connection between the network and each consumer.
- Add legend, title block, and voltage levels key Provide a legend defining all symbols used. Add a title block with diagram number, revision, date, network operator or project name. Include a voltage level key showing which parts of the diagram correspond to which voltage level, using colour coding if the diagram format permits.
Specifications
| Nominal LV supply voltage (IEC countries) | 230 V AC single-phase / 400 V AC three-phase, 50 Hz (IEC 60038) |
|---|---|
| LV voltage tolerance (IEC 60038 / EN 50160) | ±10% of nominal (207–253 V single-phase in EU/UK/IEC countries) |
| Typical MV distribution voltages (IEC regions) | 6.6 kV, 11 kV, 22 kV, 33 kV — region-specific; 11 kV most common in UK and Commonwealth countries |
| Nominal LV supply voltage (North America, residential) | 120/240 V AC split-phase, 60 Hz (single-family); 120/208 V AC three-phase wye for commercial |
| Standard distribution transformer vector group (IEC, 11 kV / 400 V) | Dyn11 (delta primary, star secondary with neutral, 30-degree phase shift) |
| IEC system earthing arrangements | TN-S, TN-C, TN-C-S, TT, IT — defined in IEC 60364-1; TN-C-S (PME / multiple earthed neutral) is most common in UK/Commonwealth LV networks |
| Nominal supply frequency | 50 Hz (IEC regions: Europe, UK, Africa, Asia-Pacific); 60 Hz (North America, parts of South America and Japan) |
Safety warnings
- Electricity distribution infrastructure — including substations, overhead lines, and underground cables — is operated by licensed network operators. Under no circumstances should unauthorised personnel approach, enter, or work on distribution infrastructure. All work on distribution networks must be performed by authorised and trained personnel under a permit-to-work (PTW) or safe system of work (SSoW) approved by the network operator. This content is for educational and reference purposes only.
- High-voltage electrical equipment is lethal at the voltages and fault currents present in distribution systems. Step and touch potential hazards exist even at significant distances from energised equipment. Safety distances mandated by the applicable regulations (IEC 60900, IEEE 516, or local equivalents) must always be observed.
- Before designing any system that connects to or interfaces with the distribution network (including embedded generation, EV charging infrastructure, or energy storage), obtain formal approval from the relevant distribution network operator (DNO) or utility. Unauthorised connection to the public distribution network is illegal and dangerous.
- When working near underground distribution cables, always obtain and verify the locations of all buried services before any excavation using the applicable dial-before-you-dig service or cable location service in your country. Striking a live underground distribution cable is potentially fatal.
- All electrical installations connected to the public distribution network must comply with the applicable wiring regulations: NEC / NFPA 70 (USA), BS 7671 (UK), AS/NZS 3000 (Australia/New Zealand), SANS 10142 (South Africa), or IEC 60364 (international), and must be inspected and tested by a qualified electrician before energisation.
Tools needed
- Single-line diagram (SLD) drawing software or CAD application (for planning and documentation)
- Power system analysis software (for fault level, load flow, and protection coordination studies)
- High-voltage testing equipment (for commissioning — authorised personnel only)
- Insulated personal protective equipment (PPE) rated for the working voltage
- Gas detector (for SF₆ equipment maintenance environments)
- Cable location equipment (for underground cable identification before excavation)
- Thermal imaging camera (for detecting hotspots in energised equipment — specialist use only)
- Protection relay test set (for verifying relay settings and operation)
Common mistakes
- Confusing the single-line diagram with a three-phase wiring diagram — a single-line diagram represents all three phases with a single conductor symbol for clarity; it does not mean the system is single-phase.
- Not showing the earthing arrangement on distribution diagrams — the TN/TT/IT classification shown on the diagram has direct implications for consumer installation design and protection relay settings downstream.
- Omitting protection relay details — a distribution diagram without protection relay types, settings, and grading margins is incomplete for engineering purposes; it cannot be used for protection coordination studies.
- Showing an incorrect or unverified fault level at the consumer connection point — the prospective short-circuit current (PSCC) at the meter point determines the minimum breaking capacity of the consumer's main protection device. Using an estimated or outdated value can result in an undersized protective device.
- Drawing an SLD that does not reflect the current as-built condition — distribution networks are modified over time; an outdated SLD that does not reflect current switching arrangements or equipment changes can lead to dangerous errors during switching operations.
- Not indicating whether feeders are overhead or underground — the fault characteristics, protection requirements, and maintenance procedures differ significantly between overhead and underground sections.
Troubleshooting
- Consumer experiences frequent supply interruptions correlated with weather events
- Cause: Overhead MV feeder experiencing transient faults (vegetation contact, bird strikes, lightning) that cause the substation protection to operate; recloser may not be fitted or not operating correctly on the affected feeder Fix: This is a distribution network operator issue. Report the pattern to the DNO with dates, times, and duration of interruptions. The DNO's fault records and SCADA system can identify the feeder and fault location. The solution typically involves trimming vegetation clearances, installing a recloser, or sectionalising the feeder.
- Significant voltage deviation at the consumer's supply point (too high or too low compared to the nominal supply voltage)
- Cause: Excessive voltage drop on the LV network due to heavy loading or undersized conductors, or voltage rise on a feeder with significant embedded generation (solar PV); alternatively, the distribution transformer tap position may require adjustment Fix: Measure the supply voltage at the consumer meter point over at least 24–48 hours using a calibrated power quality analyser. If the voltage is consistently outside the statutory tolerance band, report the measured data to the distribution network operator. The DNO can adjust the transformer tap setting, reinforce the LV network, or implement other voltage management measures.
- Protection relay at the distribution substation operates spuriously (false trips without an apparent fault)
- Cause: Incorrectly set protection relay, current transformer (CT) saturation during high inrush events, external interference, or relay at end of service life Fix: This is a specialist protection engineering task requiring the substation to be de-energised under a permit to work. The relay settings must be verified against the protection coordination study. CT performance must be checked against the burden and symmetrical fault current. A protection relay test set is used to verify relay operation at the set thresholds. Do not adjust protection relay settings without a valid coordination study.
Frequently asked questions
What is the difference between transmission and distribution in an electricity network?
Transmission refers to the bulk transfer of large amounts of electrical power over long distances at very high voltages (typically 132 kV to 765 kV), where high voltage reduces current and therefore reduces resistive losses in the conductors. Distribution refers to the local delivery of power from transmission substations to end consumers at medium and low voltages. The boundary is generally defined by the network operator or regulator in each region.
What voltage does the distribution network deliver to homes?
In IEC-standard countries (UK, Europe, Australia, South Africa, and most of the world), the standard single-phase supply voltage is 230 V at 50 Hz, derived from a three-phase 400 V LV network. In North America, the standard single-phase residential supply is 120 V at 60 Hz (with 240 V available as a split-phase system). These are nominal voltages — actual voltage at the point of delivery varies within regulated tolerance bands.
What is a distribution substation?
A distribution substation is a facility containing one or more transformers that step medium-voltage distribution voltage down to low voltage for local supply. It also contains protection equipment (fuses, circuit breakers, or reclosers), switching devices, and metering. Physical forms include pad-mounted kiosk substations (ground-level enclosures in urban areas), pole-top transformers (mounted on wooden or concrete poles in rural areas), and underground network substations (in dense urban areas).
What does the earthing arrangement shown on a distribution diagram mean?
The earthing arrangement describes how the system neutral is earthed and how the protective earth (PE) relates to the neutral in the distribution network and at consumer premises. Common arrangements in IEC 60364 include: TN-S (separate neutral and PE throughout), TN-C-S (combined PEN in the network, split to separate N and PE at the consumer), and TT (neutral earthed at the transformer; consumer earth independent). The earthing arrangement shown on the distribution diagram determines the design of downstream consumer installations.
What is a single-line diagram in the context of electricity distribution?
A single-line diagram (SLD) is a simplified representation of the distribution system in which three-phase conductors are shown as a single line. It shows the connections between transformers, circuit breakers, feeders, and loads without the complexity of drawing all three phases separately. The SLD is the standard diagram format used by distribution network operators for planning, protection studies, and operator communication.