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Read MoreA pad mounted installation moves the substation out of the building and into the street, the car park or the landscaping, which is why it has become the default arrangement wherever overhead lines are not acceptable. Pad-mounted transformer and pad-mount switchgear assemblies put the equipment at surface level in a tamper-resistant enclosure, fed by underground cable instead of overhead conductors.
The result is a cleaner public realm and a network far less exposed to weather and vehicles — but the engineering emphasis moves from the conductors to the cable system, the enclosure and the earthing. This guide covers where pad-mount arrangements are used, what the switchgear options are, and the underground distribution design rules that determine whether the installation is reliable for decades. The wider underground distribution solutions offered for these projects follow the same logic.
What a Pad-Mounted Arrangement Is
A pad-mounted installation is a surface-level assembly of electrical equipment on a concrete pad or steel foundation, connected to the network by underground medium-voltage cables. The enclosure is normally compartmented and tamper-resistant, designed to be operated with the doors closed wherever possible, and finished to sit acceptably in a public environment.
Two configurations dominate. In the first, the pad hosts a transformer and its protection only, frequently with a loop facility to allow the cable system to continue past it. In the second, the pad hosts a combined assembly — switchgear, transformer and often low-voltage distribution — which is effectively a compact substation placed outside. The choice depends on whether the low-voltage distribution can be accommodated indoors or must also be moved out to the pad.
Where Pad-Mount Arrangements Are Used
- Residential and mixed-use developments: underground distribution with equipment sited in landscaping rather than on poles.
- Commercial and retail sites: distribution close to the load without consuming lettable floor area.
- Industrial parks and campuses: looped networks where pad-mounted units provide sectionalising and transformer positions around the site.
- Public realm and transport infrastructure: where appearance and public safety rule out overhead plant.
- Coastal and flood-exposed areas: subject to foundation level and water ingress design.
- Charging hubs and renewable generation blocks: small loads served by a single compact pad-mounted unit.
Pad-Mounted Switchgear Options
| Option | Switching and protection | Where it is the right answer |
|---|---|---|
| Ring main unit with load break switches and fuses | Loop switching plus fuse protection on the transformer feeder | Standard distribution pads with a single transformer and moderate fault level |
| Ring main unit with circuit breakers | Loop switching plus relay-based protection with adjustable settings | Where coordination with upstream devices must be precise or fault levels are high |
| Sectionalising pad | Loop switching with no transformer | Building the ring through a site so a cable fault can be isolated without interrupting customers |
| Combined switchgear, transformer and LV pad | Full distribution function in one enclosure | Compact sites where LV distribution cannot be housed indoors |
| SF6-free or sealed compact unit | As above, with sealed gas-free insulation | Where appearance, footprint or environmental reporting favour a sealed design |
Compact sealed assemblies are common in this application because they reduce the enclosure size needed to achieve the required clearances and because the live parts are protected from water, dust and wildlife. A unit such as a ring main unit with load break switches and fuses covers the standard distribution pad, and a 10 kV ring main unit does the same job where the network is designed around 10 kV. Where the connection point has higher fault levels, a breaker-based assembly takes over, and the wider medium and high voltage switchgear range covers both. Air insulation remains the default wherever there is room for it; the comparison of AIS and GIS switchgear sets out when a sealed design is worth the premium.
Underground Distribution Design Rules
Once the conductors go underground, the failure modes change. Overhead faults are usually transient and visible; cable faults are permanent, harder to locate and more expensive to repair. Design accordingly:
- Loop rather than radial where continuity matters: a ring allows a faulted section to be isolated while the rest of the network stays energised.
- Cable sizing for the worst case: current rating after derating for soil thermal resistivity, burial depth, grouping and ambient ground temperature — not the tabulated free-air value.
- Fault level and protection coordination: cable and joints must withstand the system fault current for the clearing time, and the protection must clear the fault without disturbing healthy sections. On a fuse-protected transformer feeder it is the characteristic of the HRC fuse or fuse link that sets that coordination.
- Jointing quality: cable joints are the most failure-prone element of the system, so the specification, materials and installer competence all matter.
- Water management: sealed cable entries, entry from above the flood level where possible, and drainage that keeps standing water away from terminations.
- Earthing: a designed earth electrode arrangement with bonding of all metalwork, cable sheaths and the enclosure, tested after installation rather than assumed.
- Fault location and indication: provisions for cable fault location and, on larger networks, fault passage indicators so that the faulted section can be found without repeated switching.
The Transformer in a Pad-Mount Arrangement
Where the pad hosts a transformer, its selection follows the installation rather than the other way round. Three considerations usually decide the outcome.
- Fire and environmental strategy: oil-immersed units need oil containment and, depending on siting, fire separation; dry-type units avoid liquid containment but are larger and more expensive at higher ratings.
- Thermal performance in a closed enclosure: the transformer’s losses have to be rejected through the enclosure, so the ventilation or cooling arrangement and the local summer ambient set the usable rating. A unit rated for an indoor temperate ambient may need derating on a sun-exposed pad.
- Load characteristics: where the load is dominated by electronic equipment — charging, IT, variable-speed drives — the harmonic content causes additional winding heating, so a suitably rated unit is specified rather than a standard distribution transformer.
Because the transformer and the switchgear are often supplied as separate items, the interface between them — primary connections, protection, earthing and the thermal coordination of the two compartments — is where responsibilities most often fall between scopes. Define it in writing.
Enclosure, Safety and Access
The enclosure is what makes a pad-mounted installation acceptable in a public place, so it carries more responsibility than an indoor cubicle does. Specify an ingress protection rating appropriate to the exposure, materials and coatings that will resist the local atmosphere, ventilation that keeps the interior within the equipment’s ambient range without admitting dust and wildlife, and locking arrangements that prevent unauthorised access while still allowing utility staff to operate the equipment safely. Where personnel may be exposed to an internal arc, arc-rated construction with vents directed away from access routes is a requirement rather than a nicety.
Access planning extends beyond the doors: working space for operating equipment in protective clothing, a pad kept clear of planting and parked vehicles, and a safe route that does not cross traffic lanes. Where the pad is public-facing, vehicle impact protection is normally part of the design.
Selection and Procurement Checklist
- Network arrangement: radial or loop, number of ways required on the ring, and whether the pad is also a transformer position.
- Voltage, current and fault level: rated voltage, continuous current and short-circuit withstand at the point of connection.
- Protection and metering: functions required, settings philosophy, and whether revenue or operational metering is needed.
- Transformer interface: rating, type, vector group, taps, protection, earthing and thermal coordination — with a clear owner for each.
- Enclosure: IP rating, material, finish, compartment arrangement, locking and arc-rated construction where applicable.
- Cable entry and termination: number, size and direction of entries, and the type of terminations the installer will use.
- Earthing and foundation: earthing terminals, electrode design requirements, pad dimensions, weight and drainage.
- Environment: temperature range, flood level, corrosion risk, and tamper or impact protection requirements.
- Documentation: type and routine test certificates, drawings, protection settings and maintenance instructions.
Conclusion
A pad-mounted solution trades overhead exposure and building space for a cable system, an enclosure and a stricter discipline about earthing and drainage. Decide the network arrangement first, choose the switchgear configuration from the fault level and coordination requirements, then treat the transformer, the enclosure and the underground cable system as one design rather than three separate purchases. That is what turns a pad in a car park into an asset that goes twenty years without a call-out.
Send us the network arrangement, fault level, load schedule and site conditions, and we will advise on the pad-mounted switchgear configuration — including how it compares with the compact substation and low voltage distribution options for the same site.
FAQ
What is a pad-mounted transformer?
It is a distribution transformer installed at ground level in a tamper-resistant enclosure on a concrete or steel pad, connected to the network by underground medium-voltage cables and usually located close to the load it serves. Pad mounting is standard where overhead distribution is not acceptable.
What is padmount switchgear?
The switching and protection assembly installed in the same kind of surface-level enclosure — typically a ring main unit with load break switches and fuses for a standard distribution pad, or a breaker-based assembly where fault levels or coordination requirements are higher.
Why is underground distribution used instead of overhead lines?
Mainly for appearance, safety and resilience. Underground cable removes visual intrusion, reduces exposure to wind, ice and vehicle damage, and removes the conductor from public reach — at the cost of higher installation expense and permanent, harder-to-locate faults.
Is pad-mounted equipment suitable in coastal or flood-prone areas?
It can be, with design provisions: a foundation above the expected flood level, sealed cable entries from above, corrosion-resistant materials and coatings for salt exposure, and drainage that keeps standing water away from terminations and cable boxes.
