Earthing Nuts, Background and Technical Description
The Earthing Nut represents a major leap forward in terminating and earthing metal cable glands associated with cables such as SWA, Mineral Insulated and braided flexible.
In most cases, metallic cable glands form a vital part of the protective connection to a cable’s metal sheath, armouring or braid. Where this connection is designed to be current carrying, it is seldom put to full test until a fault occurs. It is then required to withstand potentially heavy fault currents without failure until the protective device clears the fault.
Traditionally, such cable glands have been connected to earth either via the metal wall of the enclosure, where this is of the heavy cast threaded type, or, in the case of sheet metal enclosures, via a brass earth ring or ‘banjo washer’. This banjo washer is tightened, together with the enclosure wall, in a sandwich by the gland body and locknut. A protective conductor is then attached using a nut and bolt through a drilled hole, again sandwiching the banjo washer and the enclosure wall.
This method forms a satisfactory connection only when fully tightened through a thoroughly cleaned and de-burred, close fitting clearance hole in the metal enclosure wall.
When terminating to non-metallic enclosures in this manner, the plastic will be included in the sandwich. Under fault conditions, this plastic will deform or melt with the heat generated, allowing the joint to become loose causing more heat, greater resistance and arcing. In severe conditions this could lead to the earth connection failing altogether and leaving downstream exposed metalwork live having ‘blown clear’ instead of operating the protective device.
It is therefore essential when terminating metal cable glands to ensure a high integrity protective connection suitable for the levels of fault current expected. There are various ways of achieving this, from sandwiching the banjo between two lock nuts to brazing the banjo onto the gland, or soldering a ‘pig tail’ direct to it. All are perfectly acceptable, however they are fiddly and time consuming. Variations in enclosure design, different coatings and environmental conditions mean that it can often be difficult to obtain a lasting connection of sufficient tightness to survive large fault currents. The thinness of the locknuts themselves presents problems with tightening, causing spanners and grips to slip off with potential for damage to equipment within the enclosure or injury to the installer. It is also necessary to drill a hole for the nut and bolt through the enclosure wall. In doing this, the enclosure IP rating is compromised, there is again potential for damage and personal injury and a suitable nut and bolt needs to be provided.
Earthing Nuts have been designed by an electrician to overcome all the problems associated with the traditional banjo washer and locknut. This is achieved by a combination of clever design and quality machining. The Earthing Nut is purpose made from brass and replaces the banjo and locknut. During fitting it is securely locked against the gland thread with a grubscrew, providing a terminal to directly attach a suitable protective conductor. The Earthing Nut removes the box wall from the connection, enabling metal glands to be fitted to any thin walled enclosure. This now opens up more possibilities for using plastic enclosures where previously only metal ones were found suitable due to the metal glands being fitted.
The design also includes a number of cutting teeth formed on one surface of the nut. These teeth cut into the enclosure wall, preventing the nut from turning and enabling the gland to be tightened to the nut from the outside with only one tool. Where this feature is not desired, the nut may be reversed and tightened to the gland instead.
Earthing Nuts can be used with any metal cable gland in the range 20mm to 75mm and are available in these sizes as standard. Larger section nuts are available for single cables to accommodate the larger protective conductor lugs used. Other non-standard sizes may be catered for by special request.
Earthing Nuts and EMI / RFI
Where signal and other cables require shielding against Electro-magnetic effects, it is necessary, where they pass through a metal plate, for the screen voltage to be equalised locally with that of the plate. The Earthing Nuts’ cutting teeth may be used to quickly remove paint from metal boxes by rotating ninety degrees with a spanner while hand tight. Once the nut is fitted as normal, a local equalising connection is effected through the teeth to the enclosure.