Premium Membership ♕

Save 50% on all Video Courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles and guides.

Home / Technical Articles / ABB – AIS UniGear ZS1 design features
ABB - AIS (air insulated switchgear) UNIGEAR ZS1
ABB - AIS (air insulated switchgear) UNIGEAR ZS1

UniGear ZS1 is medium voltage switchgear with a metal enclosure, suitable for indoor installations. Metal partitions segregate the compartments from each other and the live parts are air-insulated. Since the switchgear is highly modular type, this make simple selection of components required by any application.

The functional units of the switchgear are guaranteed arc proof in accordance with the IEC 62271-200 Standards, appendix AA, class A accessibility, criteria 1 to 5.

All the installation, operation and maintenance operationscan be carried out from the front of the unit. The switchgear and the earthing switches are operated from the front with the door closed. The switchgear can be back to wall installed.


Unit compartments
Unit compartments: A-Circuit-breaker compartment; B-Busbar compartment; C-Cable compartment; D-Low voltage compartment; E-Compact gas duct channel

Each switchgear unit consists of three power compartments: circuit-breaker [A], busbars [B] and cables [C]; please refer to figure 1. Each unit is fitted with a low voltage compartment [D], where all the auxiliary instruments are housed. Arc-proof switchgear is normally provided with a duct [E] for evacuation of the gases produced by an arc; different types of gas ducts are available.

All the compartments are accessible from the front and maintenance operations can correctly carried out with the switchgear installed up against a wall. The compartments are segregated from each other by metallic partitions.

Main busbars

The busbar compartment contains the main busbar system connected to the upper isolating contacts of the circuitbreaker by means of branch connections.

The main busbars are made of electrolytic copper. For ratings up to 2500 A, the busbars are flat bars; while for currents between 3150 A and 4000 A, a special D-shape busbar is used. The busbars are covered with insulating material.

There is a single busbar compartment along the whole length of the switchgear up to 31,5 kA, which optionally can be divided into compartments. For 40/50 kA these bushings are a standard feature.

Cable connections

The cable compartment contains the branch system for connection of the power cables to the lower contacts of the circuit-breaker. The feeder connections are made of electrolytic copper and they are flat busbars for the whole range of currents.

For 17.5 and 24 kV they are covered with insulating material.

Earthing switch

Cable compartment can be fitted with an earthing switch for cable earthing. The same device can also be used to earth the busbar system (measurements and bus-tie units). It can also be installed directly on the main busbar system in a dedicated compartment (busbar applications). The earthing switch has short-circuit making capacity. Control of the earthing switch is from the front of the switchgear with manual operation, and, optionally, can also be motor operated.

The position of the earthing switch can be seen from the front of the switchgear by means of a mechanical coupled indicator.

Earthing busbar

The earthing busbar is made of electrolytic copper and it runs longitudinally throughout the switchgear, thereby guaranteeing maximum personnel and installation safety.

Insulating bushings and shutters

The insulating bushings in the circuit-breaker compartment contain the contacts for connection of the circuit-breaker with the busbar compartment and cable compartment respectively. The insulating bushings are of single-pole type and are made of epoxy resin. The shutters are metallic and are activated automatically during movement of the circuit-breaker from the racked-out position to the operation position and vice versa.


Single and three-core cables up to a maximum of twelve per phase can be used depending on the rated voltage, the unit dimensions and the cable cross section. The switchgear can be back to wall installed as the cables are easily accessible from the front.

Gas exhaust duct

The gas exhaust duct is positioned above the switchgear and runs along its whole length.

Each power compartment is fitted with a flap on its top surface. The pressure generated by the fault makes it open, allowing the gas to pass into the duct. Evacuation from the room of the hot gases and incandescent particles produced by the internal arc must normally be carried out. The UniGear ZS1 switchgear can be fitted with a complete range of solutions to satisfy all requirements, either in the case where evacuation is possible directly at the end of the switchgear, or when solutions from the front or rear are requested.

Some installations, such as marine applications, do not allow evacuation of the gases to the outside of the room and therefore a dedicated solution has been developed to guarantee personnel safety and conformity with the Standards, such as longitudinal evacuation chimneys.

Busbar applications

Each switchgear unit can optionally be fitted with an accessory busbar application:

  • current or voltage transformers for busbar measurements.
  • busbar system earthing switch.
  • top entry duct or cables to make interconnections between different sections of switchgear.
ABB medium voltage AIS Unigear ZS1
ABB medium voltage AIS Unigear ZS1
SOURCE: ABB – UniGear ZS1 catalogue

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Ephraim
    Feb 04, 2015

    How have you been, i would like to perform a PD and Thermography test on UNGEN ZS1 switch gear Busbar compartment. How can i do the check since i am unable access any opening on the Live busbars, should i use the the flap, advise please….

  2. walaa elsayed
    Jan 14, 2015

    Item Description Qty Unit
    1 Lower flower box 9 Each
    2 Upper flower box 5 Each
    3 Isolate for cable support 3 Each
    4 Bushbar bushing 6 Each
    This spare parts will be used for: 24 KV switchgear
    Type: UNIGEAR ZS1
    Manufacturer: ABB

  3. matt
    Jan 21, 2014

    Can you please tell me in regards to the equipment can the VT’s in the UniGear ZS1 be racked out off a live bus please

Leave a Reply to matt

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

six  ×    =  twelve

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge