A pole-mounted substation is a distribution transformer mounted on one or two poles and connected to an overhead medium-voltage line, with fuse cutouts, surge arresters and grounding. It is the simplest solution for small capacities. The pole strength required depends on the transformer power and weight and is set by the local code and the utility's construction standard, which also require overcurrent and surge protection on the primary side.
What a pole-mounted substation is
The pole-mounted substation is the simplest way to go from the overhead medium-voltage line to a customer’s low voltage. The transformer hangs from a pole and is protected with fuses and arresters. On the low-voltage side, the cable runs down to the meter and the panel. It needs no room, switchgear or enclosure, as long as the safety clearances are respected.
It is part of the family of substations a plant can have, together with the pad-mounted, the compact switchgear one and the conventional one; the differences are compared in industrial substation.
It is used on farms, small industries, shops, pumping stations, service stations and on utility networks to supply neighborhoods. It is often the first substation a customer meets when leaving low voltage.
Parts of a pole-mounted substation
| Part | Function |
|---|---|
| Pole and structure | Support the transformer and the line; crossarms, hardware and clamps sized for the weight and the loads |
| Fuse cutouts | Protect the transformer and the line against overcurrents; allow opening with a hot stick to isolate it |
| Surge arresters | Limit the lightning and switching overvoltages that arrive over the overhead line |
| Transformer | Single-phase or three-phase, normally oil-filled, with its pole mounting bracket |
| Low-voltage service drop | Carries the energy to the customer’s panel or metering equipment |
| Grounding | Connects the neutral and the tank to earth and gives a path to fault and discharge currents |
The transformer is a MV/LV distribution transformer, and its design for pole mounting has its own standard in the North American market: IEEE C57.12.20 covers overhead-type distribution transformers of up to 500 kVA and 34.5 kV on the high-voltage side. The product must also meet the transformer requirements of the local code and of the utility.
Up to what capacity: what limits it
There is no single maximum power for a pole; what is limited is the mechanical strength of the assembly:
- The pole must have a breaking load (ultimate strength) sufficient for the transformer weight. Local codes and utility standards usually give tables by transformer power and weight.
- For other configurations and combinations of poles, such as double-pole structures, a loading analysis is needed to guarantee the stability of the assembly, the transformer and its components.
- The fixing hardware must support a safety factor over the transformer weight, as set by the applicable standard.
- In public urban space, double-pole structures are best avoided because of visual and mobility impact.
In addition, each utility defines in its construction standard which capacities it accepts on a pole and when it requires a pad-mounted or switchgear substation. Beyond what fits on a pole, the plant moves to a pad-mounted substation or to a compact substation. The design is adapted to the local code and to the utility’s requirements that the client gives us.
How it is protected
Every pole-mounted substation should have, at least on the primary side, overcurrent and overvoltage protection. In practice:
- Fuse cutouts with expulsion fuses on every medium-voltage phase. The fuse rating comes from a coordination study and must protect the transformer; the utility publishes the table of fuses it accepts. See medium-voltage fuse.
- Surge arresters in the path of the impulse current and as close as possible to the transformer bushings. A long connection between the arrester and the bushing greatly reduces the protection. See surge arrester.
- Grounding: the neutral point and the transformer case solidly connected to earth, and the grounding system in line with the applicable code; the grounding grid explains how step and touch voltages are controlled.
- Low voltage: the main breaker or fuses of the customer’s panel, coordinated with the medium-voltage fuse.
The medium-voltage connections must be rigid enough that wind or vibration does not bring them close to parts that must not be energized.
Advantages and limits
Advantages: little civil work, fast installation, no room or oil containment pit (pole-mounted substations are generally exempted from the pit, subject to local code) and no enclosure if the safety clearances are met.
Limits:
- Capacity limited by the mechanical mounting and by the utility’s standard.
- Protection with fuses only: there is no relay and no fine selectivity with the plant.
- Exposed to weather, pollution and lightning.
- Maintenance with the line energized nearby, which requires coordinated switching and qualified personnel.
- If the plant grows, changing the substation means a new procedure with the utility.
That is why, in a plant that expects to grow, it is worth comparing from the design stage the total cost of a pole-mounted substation with that of a pad-mounted or switchgear one. The first is cheaper to build, but may soon fall short and force a rebuild of the service entrance, the metering and the protection.
Testing and maintenance
Before energizing, the transformer is tested like any distribution unit: insulation resistance, turns ratio, winding resistance and verification of the vector group and the no-load tap changer position. The ground resistance is also measured and the installed arresters and fuses are checked.
In operation, the connections, fuses, arresters after storms, oil leaks, tank corrosion and the condition of the pole and hardware are inspected. When the transformer belongs to the utility, its maintenance may be included in the distribution network maintenance plan; when it belongs to the customer, the plan is the plant’s.
How we help
At Tiza Group our team designs substations for industrial plants, from a pole-mounted one to medium-, high- and extra-high-voltage substations: capacity selection, fuse and protection coordination, grounding and documentation for the utility. We also test the transformer before energizing, in Colombia or elsewhere on request.
See our engineering design and studies service and contact us to review whether your plant fits a pole-mounted substation or needs another solution.
Related guides
- Industrial substation: parts, types and protections
- Pad-mounted substation: what it is and when to use it
- MV/LV distribution transformer
- Medium-voltage fuse
- Surge arrester
- How many kVA does your plant need
- Ground resistance test
References
- IEEE C57.12.20-2023, Standard for Overhead-Type Distribution Transformers 500 kVA and Smaller; High Voltage, 34 500 V and Below; Low Voltage, 7970/13 800Y V and Below.
Frequently asked questions
What are the parts of a pole-mounted substation?
The pole or poles with their crossarms and hardware, the medium-voltage fuse cutouts, the surge arresters, the transformer with its mounting bracket, the low-voltage service drop to the panel or the meter, and the grounding of the neutral and the tank.
Up to how many kVA can a transformer be installed on a pole?
It depends on the strength of the pole and the hardware, the transformer weight and the construction standard of the utility. IEEE C57.12.20 covers pole-type distribution transformers up to 500 kVA, but in practice a single pole carries the smaller sizes, and for other pole arrangements a loading analysis is required. Each utility defines which capacities it accepts on a pole.
What protection does a pole-mounted substation have?
At a minimum, overcurrent and surge protection on the primary side of the transformer: fuse cutouts and surge arresters installed as close as possible to the bushings. On the low-voltage side, the breaker or fuses of the panel protect the plant circuits.