A recent safety notice from the Health and Safety Executive (HSE) has highlighted the risks that high humidity can pose inside substations, following the failure of GEC Type VMX high-voltage switchgear at a COMAH site.
While the HSE identified partial discharge as the most probable cause of the failure, the notice also draws attention to the environmental conditions surrounding the equipment.
Water ingress through cable ducts, basements and below-ground penetrations is a common contributor to damp substation environments. If those entry points are poorly sealed or have deteriorated over time, moisture can build up and make humidity harder to control.
For CSD, this is an important part of the wider asset-protection picture. Effective cable and pipe sealing is not only about keeping floodwater out, it also helps maintain the internal conditions that critical electrical equipment depends on.
High humidity and partial discharge
Around two years before the failure, partial discharge activity had been detected on the circuit breaker. At the time, relative humidity within the substation was recorded at 71% at 24°C.
Fourteen months before the incident, partial discharge was detected again, with relative humidity recorded at 81% at the same temperature.
The manufacturer’s instructions referenced by the HSE state that relative humidity should not exceed 80%, while conditions that allow condensation to form on or within the equipment should also be avoided.
High humidity can contribute to conditions that increase the likelihood of surface discharge, particularly where contamination is present. Condensation can also lead to corrosion and deterioration of insulation over time.
Managing the environment around switchgear is therefore an important part of maintaining the equipment itself.
How is moisture getting into the substation?
Humidity within a substation can have several causes, but water ingress into cable basements, trenches and underground ducts is a common area to investigate.
These parts of the building are particularly vulnerable because they are often below ground level and exposed to groundwater, surface water and changing external conditions.
Cable ducts can provide a direct route into the building if they have not been properly sealed.
A damaged, deteriorated or unsuitable seal may allow relatively small amounts of water into a cable basement over a long period, it doesn’t have to be off the back of a flood. The result can be standing water, damp conditions and consistently elevated humidity within the substation.
This can become more difficult to manage as substations age and cable arrangements change. New cables are installed, redundant cables are removed and existing penetrations are reopened as part of upgrade and maintenance programmes.
Unless the sealing around those penetrations is reinstated correctly, another potential route for water ingress is created.
Controlling moisture at source
Heating, ventilation and dehumidification can help manage conditions inside a substation, but they do not address the route by which water is getting in.
Cable ducts and below-ground penetrations are common weak points. If these are inadequately sealed, groundwater can pass into cable basements and chambers, creating damp conditions that are much harder to control.
This is where the sealing system becomes an important part of the substation’s wider protection strategy. Cable and pipe penetrations need a reliable watertight seal that can withstand groundwater pressure and continue to perform over the long term.
They also need to allow for future changes. Substations are regularly upgraded, with cables added, removed or replaced as network requirements evolve. Re-enterable sealing systems allow those changes to be made without compromising the integrity of the penetration or requiring the entire seal to be replaced.
For asset owners, this means not just managing humidity once it appears, but to reduce one of its potential sources by keeping water out of the building in the first place.
Looking beyond the switchgear
The HSE notice advises operators of affected VMX equipment to assess its condition, review maintenance arrangements and establish whether the environmental conditions meet the manufacturer’s requirements.
It also recommends partial discharge surveys alongside measurements of relative humidity and ambient temperature.
Where consistently high humidity is identified, the condition of the surrounding building should be part of that assessment.
Cable basements, chambers, ducts and cable entry seals can all provide useful evidence. Standing water, damp areas, deteriorated sealing materials or signs of previous ingress may point to an underlying problem that needs addressing.
Existing seals also need to be considered as part of ongoing maintenance. A penetration that was correctly sealed when a substation was built may have been altered several times since then.
Keeping substations dry is part of asset protection
The HSE safety notice relates specifically to GEC Type VMX switchgear manufactured between 1985 and January 2003, and operators with affected equipment should follow the actions set out by the HSE.
The issue it raises has relevance across the whole electricity network.
Switchgear performance cannot be considered separately from the environment in which it operates. Water ingress, humidity, temperature and contamination can all affect conditions around critical electrical equipment.
With substations being asked to operate for longer while accommodating new connections, increased capacity and ongoing upgrade programmes, maintaining those conditions becomes increasingly important.
Keeping water out of cable basements and below-ground penetrations is one part of that work. Done properly, it helps protect the environment around the equipment and removes a preventable source of moisture from the substation.