<P> A mobile substation is a substation on wheels, containing a transformer, breakers and buswork mounted on a self - contained semi-trailer, meant to be pulled by a truck . They are designed to be compact for travel on public roads, and are used for temporary backup in times of natural disaster or war . Mobile substations are usually rated much lower than permanent installations, and may be built in several units to meet road travel limitations . </P> <P> Substations generally have switching, protection and control equipment, and transformers . In a large substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network . Smaller distribution stations may use recloser circuit breakers or fuses for protection of distribution circuits . Substations themselves do not usually have generators, although a power plant may have a substation nearby . Other devices such as capacitors and voltage regulators may also be located at a substation . </P> <P> Substations may be on the surface in fenced enclosures, underground, or located in special - purpose buildings . High - rise buildings may have several indoor substations . Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions . </P> <P> A grounding (earthing) system must be designed . The total ground potential rise, and the gradients in potential during a fault (called touch and step potentials), must be calculated to protect passers - by during a short - circuit in the transmission system . Earth faults at a substation can cause a ground potential rise . Currents flowing in the Earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution . Where a substation has a metallic fence, it must be properly grounded to protect people from this hazard . </P>

When it is necessary to switch on an electrical current from a remote location