Description
WHAT HAPPENED?
Rodent damage occurred to the sheath of low voltage (LV) cables in the transformer section of a wind turbine packaged substation.
The damage was not immediately remedied because it was collectively understood (including by Senior Authorised Persons who had inspected the damage) that the visible silver-coloured wires, where the sheath had been chewed away by mice, was the wire armour or stranded metallic screen of the cable.
However, it was subsequently identified by an experienced electrical contractor that the visible wires were the exposed conductor and not wire armour.
Where the LV cables were damaged is located in a secure compartment in the packaged substation where access is restricted to authorized persons only. The identification of the hazard happened while other work was being carried out during a grid outage across the whole wind farm – i.e. during a period of electrical isolation. Nobody was harmed as a result of the incident.
WHY IT HAPPENED?
The root cause of the issue was damage caused by mice to the LV cable sheath.
However, the damage and exposed live conductor was allowed to persist due to a mis-understanding as regards the type of cable.
Lessons learned / Findings
After research into the cable specification no evidence was found in the cable specification documents to suggest a wire armour or screen and a close inspection of the actual cable by experienced cable jointers confirmed there to be none.
An assumed (false) understanding of the nature of the LV cables was allowed to persist and a lesson learned is that (a) thorough investigation should take place into the specification of cables that are damaged and/or (b) appropriate repairs should be undertaken without delay in any circumstance where a cable sheath is damaged.
As well as introducing anti-rodent measures into the packaged substation, it must be borne in mind that rodents may cause damage to cables and/or other electrical components, so that the insulation could be compromised and an electric shock hazard may exist. Appropriate care must be taken and appropriate isolation(s) applied, before handling any cable and/or component of the electrical system.
General Discussion Point
The electrical contractor who had identified the hazard proposed that a heat-shrink over sheath could be applied to repair the damaged cable sheaths and this was carried out by experienced cable jointers to effect a satisfactory repair. This effectively eliminated the hazard of the exposed conductor.
Note that the area where the LV cables were damaged is located in a secure compartment in the packaged substation where access is restricted to authorized persons only. Nevertheless, as well as an ongoing operational restriction to alert any party carrying out work, warning signs have been placed in all such areas.
In addition, all parties who have had a role in respect of Operational Management, Site Management and/or SAP services at this site who would have been aware of the situation as described herein have been provided with an incident report so that they will have been able to raise awareness within their respective organisations.