Earth Bonding | Joints & Terminations & Overheating Prevention
Published 10 Jan 2020
Earth Bonding
Earth bonding across joints and at terminations does not always receive the level of care and attention that it requires. An ineffective earth bond can be just as damaging to MV cable accessory performance as inadequate phase conductor connections. It is also possible that medium voltage electrical power network safety may be compromised if an earth connection is lost or has high resistance.
A failure mechanism frequently seen in joints and terminations is overheating at positions where the cable metallic screen and/or armour is connected across a joint or to an earth connection at a cable termination. Overheating results when a high resistance earth bond carries fault current or induced ‘circulating current’. The overheating can damage underlying cable insulation and cause insulation failure of the MV power cables.
The most frequent cause of earth bond problems is the presence of continuous induced earth circulating currents in single-core circuits. These circulating currents are induced in the metallic earth components of the cable if there is a connection to earth at each end of the cable (referred to as ‘solid bonding’).
The problem is likely to be worse in circuits with large conductor cables with aluminium wire armour (AWA). Because of the high conductivity of AWA, the circulating current can be many hundreds of Amps when the cable conductor is heavily loaded.
Unfortunately, MV cable accessory suppliers are not always made aware of the possibility of circulating currents when asked to specify products such as MV joints, terminations or connectors. The installer should be in a good position to know the system earthing arrangements and to check before installation whether the earth bonds provided with the accessory are adequate. If there is doubt, installation should not proceed.
Further Reading
- Cables | MV Paper Insulated v MV Polymeric Insulated Cables
- The Installation Site | MV Cable Joints & Cable TerminationsÂ
- First Steps | MV Joints, Jointers & Initial Considerations
- Cable Preparation | Jointing & Terminating Aspects of MV Cable Preparation
- Conductor Connectors | Crimp v Mechanical Connectors with Joints & Terminations
- MV Cable Accessory Technologies | Heat Shrink, Cold Shrink & Push-on
- MV Cables & Causes of MV Cable Failures
- MV Cables | Electric Field & Stress Control
LV, MV & HV Jointing, Earthing, Substation & Electrical Eqpt
Thorne & Derrick International are specialist distributors of LV, MV & HV Cable Installation, Jointing, Duct Sealing, Substation & Electrical Equipment – servicing UK and global businesses involved in cable installations, cable jointing, substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.
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