What is Leakage Current?
Leakage current is the current that flows from either AC or DC circuit in an equipment to the chassis, or to the ground, and can be either from the input or the output. If the equipment is not properly grounded, the current flows through other paths such as the human body. This may also happen if the ground is inefficient or is interrupted intentionally or unintentionally.
Where does the leakage current flow
The leakage current in an equipment flows when an unintentional electrical connection occurs between the ground and an energized part or conductor. The ground may be the reference point of zero voltage, or the earth ground. Ideally, the current leaking from the power supply unit should flow through the ground connection and into the installations earth ground.
Leakage current in laptops or devices using two pin plugs, is mostly through the signal cables connected to other grounded or ungrounded equipment such as printers. The other equipment provides a path to the ground if it is properly grounded or may give an electric shock to anybody touching the exposed metal parts if not properly earthed.
The leakage in devices is largely due to the imperfections in the insulators or materials that make the component such as the semiconductors and capacitors. These results in to small current leaking or flowing through the through the dielectric, in the case of a capacitor.
Leakage current in EMC filters
Leakage current in power supplies may occur due to the EMC filters, which utilizes Y capacitors between the live and neutral conductors. This causes some leakage current to flow from the neutral or the live conductor to the power supply casing which is normally connected to the earth ground.
Most power supply manufacturers specify this current which should always be lower than 3.5 mA as per the IEC-60950-1 regulations. This ensures that the current is very low and cannot harm the person who touches or comes into contact with the power supply case. A power supply with a good earth ground reduces the leakage current significantly by providing a low resistance path to the ground.
Leakage current flow in an EMC filter – Image Credit
The filter manufacturers usually specify the maximum leakage current that the filter will leak, but these are only theoretical values and the actual values may deviate from these especially if parameters such as voltage or frequency change. To get an accurate value of the leakage current, it is advisable to measure the current that flows to the ground when the filter is in operation.
Permissible maximum leakage currents
There are standards that specify maximum leakage currents that are safe for humans under different conditions. These vary with the application and type of possible contact as well as the type of ground connection.
Designers are supposed to ensure that the leakage current does not harm the users who touches the enclosure of a power supply or the powered equipment. All applications have their upper limit on the current that should flow. Medical equipment and other sensitive equipment are required to have very low currents due to the nature of their applications and impact they can have.
The standards are more stringent in medical applications since the weak patients are more vulnerable to electric shocks which can be fatal.
Typical leakage current limits by application are:
- Permanently connected – 3.5 mA or more in some applications
- Movable or pluggable, not handheld – 3.5mA
- Handheld – 0.25 mA
The permissible leakage current under normal conditions is 0.5mA and 1mA under a single fault condition. The leakage current is very dangerous if it exceeds the permissible safe limit. It is even worse in medical applications due to both the risk it poses to both patients and caregivers. Only a small current need to flow through the human body to cause harm, and can be fatal for patients whose immune systems are already weak.
Typical leakage current for different equipment classes
Class I Equipment:
Must have a protection against the electric shock by means of a basic insulation in combination with a protective earth ground connected to the equipment case.- maximum leakage current is 0.75 mA for the hand held and 3.5mA for the other equipment.
Class II equipment:
These equipment do not have a protective earth ground. Such equipment uses reinforced or double insulation to provide protection against the electric shocks. Maximum leakage current is 0.25 mA.
These are the super extra low voltage (SELV) circuits in which there are no hazardous voltages.
Leakage current will flow when it is not desired to do so, either due to poor design, failure grounding or insulation in equipment, imperfections in component materials and more. The magnitude of the current can be reduced by proper design and observing of the best standards and practices.
Different types of equipment have a permissible maximum leakage current based on application and voltage. Other than design, an effective method of reducing the leakage current is ensuring that the equipment has a proper earth ground.