Surface leakage current is an unwanted electrical current that flows along the surface of an insulator instead of in the intended conductor. It arises above all when moisture and contamination make the insulation surface conductive. The consequences range from increased energy loss and tripping residual current devices to fire hazards. This page explains how to detect and measure surface leakage current at home, how it differs from leakage current and how designers prevent it from the outset through creepage distances and tracking-resistant insulating materials (CTI value).
GOBA supplies insulation materials with high creepage current resistance for electrical engineering applications.
What is the difference between surface leakage current and leakage current?
Definition and formation of surface leakage current
Surface leakage current refers to unwanted electrical currents that flow along the surface of an insulator rather than being conducted within a conductor. This effect occurs particularly in the presence of moisture, contamination or deposits that increase the electrical conductivity of the insulation surface. The path the current takes is called the creepage path.
Characteristics and causes of leakage current
Leakage current, in contrast, describes electrical currents that flow through the insulation itself, not along the surface but through the material. This can be caused by ageing, mechanical stress or material fatigue. Leakage currents are particularly critical in low-voltage installations, as they can represent a latent fire hazard.
Effects on electrical systems and safety
Both types of current are potentially dangerous, as they:
- cause malfunctions in electrical equipment
- reduce the service life of insulating materials
- increase fire risks in electrical installations
- can cause electric shocks to people
Detecting and measuring surface leakage current at home
Typical signs of surface leakage current in the home
- The residual current device (RCD) trips repeatedly without an identifiable faulty appliance
- Sockets, switches or cables heat up unusually
- Lights flicker or there is crackling at electrical components
- Slight tingling when touching devices with metal housings
- Electricity consumption rises for no apparent reason
The RCD is the most important safety net: it compares the outgoing and returning current and disconnects the circuit as soon as more than 30 mA flow off via an unwanted path, including a creepage path.
Measuring surface leakage current: how professionals proceed
- Disconnect the circuit and unplug the affected devices.
- Use an insulation tester to measure the insulation resistance between conductors and protective earth, in residential installations at 500 V DC.
- Values below 1 MΩ indicate an insulation fault or creepage path, the circuit then belongs in the hands of a qualified electrician.
- To narrow it down, switch on circuits and devices one by one until the faulty section is found.
Important: measurements on the fixed electrical installation are the job of a qualified electrician. As a resident, however, you can do the preparatory work and observe which device or circuit trips the RCD.
Which factors promote the formation of surface leakage currents?
Influence of humidity and air moisture
High humidity and condensation promote the conductivity of surfaces. Moisture settles on insulators and forms a conductive layer that facilitates surface leakage currents. This occurs particularly in unheated or poorly ventilated areas, at home for example in cellars, bathrooms and outdoor areas.
Contamination and deposits
Dust, soot, salts or chemical reactions on insulation materials can form conductive deposits. Industrial environments with pollutant load and corrosion are particularly affected.
Importance of the surface condition of insulators
Insulators with a rough or porous surface are more likely to absorb contaminants or moisture. Smoother surfaces reduce surface leakage current formation, while materials such as silicone or glass-fibre-reinforced plastic have special hydrophobic properties that prevent surface leakage currents.
Tracking resistance and CTI value: how insulating materials are classified
Tracking resistance describes how well an insulating material resists the formation of conductive tracking paths. It is measured according to DIN EN 60112 via the CTI value (Comparative Tracking Index): a standardised electrolyte solution drips onto the material surface between two electrodes every 30 seconds, and the CTI indicates the highest voltage in volts at which the material withstands 50 drops without tracking. Based on the CTI, IEC 60664-1 divides insulating materials into four groups:
| Insulating material group | CTI value | Tracking resistance |
|---|---|---|
| I | 600 and above | very high |
| II | 400 to below 600 | high |
| IIIa | 175 to below 400 | medium |
| IIIb | 100 to below 175 | low |
The group directly determines the design: the higher the CTI value, the shorter the creepage distances according to IEC 60664-1 may be, and the more compact the component can become. When designing insulating parts, we therefore select the material according to pollution degree and required creepage distance: for standard applications, materials of group IIIa are sufficient, in humid or contaminated environments a material of group I or II pays off because it allows shorter creepage distances and thus smaller components. The CTI value can be found in the data sheet of the respective material, and we support you in the selection as part of our consulting services.
How does material fatigue affect the formation of surface leakage and leakage currents?
Ageing processes in insulating materials
Over time, insulating materials age through thermal and mechanical stress, reducing their dielectric strength. Plastics in particular lose their insulating effect through micro-cracks and degradation.
Influence of UV light and ozone on insulation materials
Long-term UV radiation and ozone can make insulating materials brittle and cracked. Overhead-line insulators in particular are strongly exposed to these conditions and require special protective measures.
Effects of operating temperature and thermal stress
Elevated operating temperatures accelerate the ageing of insulating materials. This leads to increased oxidation and degradation, impairing or destroying the insulation. The insulation class defines which continuous temperature a material can withstand.
What preventive measures exist against surface leakage and leakage currents?
Selection of suitable insulating materials and substrates
Durable materials with high moisture resistance and a high CTI value (e.g. polyester films, glass-fibre-reinforced plastic or silicone) minimise the risk of surface leakage currents.
Regular maintenance and cleaning of electrical components
Electrical installations should be cleaned and inspected regularly to remove deposits and contamination.
Optimisation of creepage distances and air clearances
Extended creepage distances and larger air clearances reduce the risk of partial discharges and surface leakage currents. Ribs, grooves and webs in insulating parts extend the creepage path without enlarging the component. In high-voltage lines in particular, creepage distance extensions are implemented through insulated covers or special insulators.
How can surface leakage and leakage currents be measured and monitored in practice?
Measurement methods and instruments for detecting surface leakage currents
Surface leakage currents are usually measured with special insulation resistance testers. A rule of thumb is: the lower the insulation resistance, the higher the risk of surface leakage currents.
Monitoring systems for leakage currents in low-voltage installations
Modern leakage current monitoring systems detect even small fault currents and can trigger alarms to prevent greater damage.
What dangers do surface leakage and leakage currents pose to people and equipment?
Risk of electric shocks and electrical accidents
An excessively high surface leakage current can electrify contact surfaces, posing a significant electric shock risk to people.
Potential damage to electrical and electronic devices
Leakage currents can damage sensitive electronic components or cause entire systems to fail.
Fire hazard from surface leakage and leakage currents
The greatest danger of surface leakage currents is potential fire formation, as local overheating or arcing can lead to a short circuit or ignition of the insulating materials.
GOBA conclusion: the CTI value sets how compact the part may be
The tracking resistance of an insulating material appears as the CTI value in the data sheet, measured to DIN EN 60112. The CTI determines the material group to IEC 60664-1, and the group determines the creepage distance your design has to maintain. That creates a lever on installation space: a group I material with a CTI of 600 V or above permits shorter creepage distances than the common group IIIa standard at 175 to below 400 V, so the part may be built more compactly. Check the CTI before material release, while the geometry is still open. Materials with documented tracking resistance are part of our material range.
