Junction Box Assembly
Author: Stefan Bichlmaier
The ORCASat ground station consists of two aluminum antennas (VHF, UHF) which are almost 20’ long. They will be mounted on top of the Engineering Lab Wing at UVic, which exposes them to the risk of lightning. In the event of a storm, huge impulses of energy from nearby lightning strikes may couple into the antennas, or lightning may even strike the antennas directly. Since these will be connected to expensive radio equipment inside the radio room which may be used by ground station operators, a method of protecting both people and equipment from the strikes is required. An established way of achieving this is establishing a zone of protection (i.e. the radio room) to which all RF and control lines enter at a single point, via adequate surge protection. To establish this for the ORCASat ground station, a special junction box designed for this purpose was selected from KF7P.
The junction box assembly contains devices called surge arresters which are specially designed in-line protection devices which shunt the electrical energy during a lightning strike to ground. Inside the surge arresters, a special formulation of gas separates the main conductor from ground. At low voltages seen during nominal conditions, the gas acts as an insulator. However, under very high voltages seen during lightning strikes, the gas ionizes and passes current to ground. This image shows a demonstration of a gas discharge tube in action.
Every single device which has an electrical connection to the tower must contain a properly grounded in-line surge arrester. There are coaxial arresters which protect RF equipment, but also DC and Ethernet arresters to protect control lines for rotors, and networked equipment. These arresters are all mounted inside the above-mentioned weatherproof box from KF7P, which is specially designed to serve as the single point ground entry point for the ground station radio room. The box contains copper plates that the surge arresters are mechanically and electrically bonded to, and the copper plates are grounded to the building site ground. With so much copper, and carefully applied antioxidant compounds, the electrical path to ground has very low impedance, which is the key prerequisite for correct operation of surge protection equipment.
As mentioned above, mounting the arresters inside the junction box requires care when bonding metallic joints together. The priority is minimizing impedance between the surge arrester and ground. This is ensured by removing copper and aluminum oxidation on conducting surfaces, cleaning them thoroughly, then applying an appropriate antioxidant compound, which is an emulsion containing small, metallic particles. The emulsion fills in microscopic defects, creating a better electrical bond, and prevents corrosion.
The junction box assembly is a key step towards ensuring the safety of ground station operators, and the longevity of the ORCASat ground station. Their installation will be covered in a blog post at a later time.