Safety Alert 91 - Electric Shock (Shared Neutral Fault)

Date
22 April 2021

Description
The apprentice and tradesperson were working at a commercial building, replacing a switchboard in a corridor. After disconnection of all the circuits from the board, the redundant cables were tested to verify de-energisation and placed into the cable tray that had been installed in the corridor ceiling void.

Several days after the de-commissioning and de-energisation of the switchboard, the apprentice was installing junction boxes to unterminated cables in the ceiling void. Whilst carrying out this task, he has inadvertently touched an unterminated end of a cable connected to the redundant lighting circuit on the cable tray, resulting in an electric shock.

After further investigation it was found that during the installation or modification of the lighting circuit in the area, two circuits from different switchboards were joined together.

As the lighting circuits were not protected by a RCD, the fault condition had not been noticed. It is believed that the fault was contributed to by the shared neutral between the circuits. When the lighting on the other switchboard circuit was turned on, it caused the neutral conductor to become live on the cable that the apprentice had come into contact with.

Safe Work Method Statements (SMWS) and pre-start had been completed. Correct PPE was in use at the time and the unterminated cable was confirmed as a circuit feed removed from de-energised switchboard.

Root cause

• The unterminated cable had been disconnected from the switchboard and tested confirmed de-energised. Several days later it had been retested to confirm de-energised and excess cable was trimmed. It was not until the day of the incident that the cable had been found to be live.
• Poor workmanship from the electrician who carried out the original installation directly contributed to the fault condition that caused the incident.
• Incorrect wiring from one zone to another - it needs to be noted that the source of the supply would have been very difficult to identify as no indication was present. It was only through further investigation by the site tradesperson that a back feed was established.

Recommendations

• EGT apprentices are to ensure the “test before you touch” principles are followed prior to commencement of all work activities.
• Always make unterminated cables “safe” to ensure live parts are not exposed if a fault condition arises.

Feedback
For further information regarding this Safety Alert, please contact EGT on (08) 6241 6100 or speak with your Field Officer.