The Ground Rules: Earthing, Class, and EMC

Jul 5, 2024
Bare feet on green grass
Simply grounding the output of a standard class II AC/DC switching power supply can create EMC problems. The solution is to use an external mains filter or to select a RECOM series that is specially designed for PELV applications with additional internal filtering.

Table of contents

  1. Why ground?
  2. What is the Power Supply Class?
  3. Grounding the input
  4. Grounding the output (Download/Login)
  5. Conclusion (Download/Login)

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Why ground?

Mains electricity is a hazardous voltage, so safeguards are needed against electric shock (Table 1). As current always takes the easiest path, earthing is one way to reduce the electric shock hazard by shunting any leakage or fault current back to the ground potential instead of through the relatively high impedance of the human body. The other way to safeguard against electric shock is to introduce sufficient insulation to block the flow of current to safe levels. Usually, for mains voltages, two separate means of insulation (double or reinforced insulation) are required so that if one insulation barrier fails, the remaining barrier(s) can still block the flow of current.
Electric current effects and safety classes
Table 1: Human body threshold currents (source: DC/DC BoK 6.1.2).
Electrical ground symbols: Signal, Chassis, Earth
Fig. 1: Types of ground symbol.
If a power supply is grounded or earthed, it will carry a standardized symbol to indicate the ground connection. There are three types of ground symbol because the function of the ground potentials are slightly different (figure 1):

Signal ground: The return path or zero voltage connection point within a circuit. This point need not be connected to the chassis or earth ground point, but it can be.

Chassis ground: A single connection to the metal chassis or enclosure. The function of the chassis ground is to collect any stray or induced voltages and return them to a ground potential (shielding function), but it can also be connected to the earth ground to prevent electric shock (grounding function). It is important that the chassis ground and earthing ground meet at a single connection point, otherwise current could flow through the chassis from one part of the circuit to another to form a ground loop. Ground loops can insert unwanted electrical noise into a circuit harming performance and adversely affecting the EMC (Electro-Magnetic Compatibility) immunity.

Earth ground: this is the Protective Earth (PE) connection that is wired back to the earth pins in the mains connector forming the safety low impedance path for any leakage or fault currents. Earth and Ground are used interchangeably in this White Paper.

What is the Power Supply Class?

It is not always necessary to ground a power supply if it has sufficient insulation to avoid electric shock to anyone who touches it or uses a safe extra-low supply voltage (SELV). Confusingly, the class system is divided into Class I, II or III, or into Class 1 or Class 2, depending on the standards used:

The IEC (International Electrotechnical Commission) circuit classifications are:
Grounded electrical circuit symbol for earth
Fig. 2a: Earth symbol.

Class I Equipment

Systems which use protective earthing (e.g., a grounded metal enclosure or grounded output) and fault supply disconnection (fuse or circuit breaker) as one level of protection and thus require only basic insulation. No exposed hazardous voltages (earthed metal enclosure or non-conducting enclosure).
Black square frame
Fig. 2b: Double insulated symbol.

Class II Equipment

The use of double or reinforced insulation to eliminate the need for a grounded metal enclosure, no exposed hazardous voltages (non-conducting enclosure). No PE connection is required, but a filter ground connection may be used (functional earth rather than protective earth).

Note: If an AC/DC power supply has a Filter Ground (FG) connection to meet the EMC regulations, it can still be classed as a Class II power supply if it does not need the ground connection for protection against electric shock.
Resistor symbol on white background
Fig. 2c: Class III symbol.

Class III Equipment

Powered from a SELV source and with no potential for generation of hazardous voltages internally, and therefore requiring only functional insulation. Functional earthing may be used, but a connection to PE is not permitted (no return path to ground via the power supply).

The US-based NEC (National Electrical Code) classification also uses a similar “Class” system to describe the different levels of protection but uses Arabic numerals to describe the level of protection against excessive energy dissipation (fire hazard).

The NEC circuit classifications are:

  • Class 1 Circuits: Power limited <1kVA and output voltage <30VAC
  • Class 2 Circuits: Power limited <100VA, input voltage <600VAC and output voltage <42.5VAC
  • Class 3 Circuits: Power limited <100VA, input voltage <600VAC and output voltage <100VAC; Additional protection against electric shock needed.

Thus, when someone talks about a “Class Two” power supply, it is important to determine if they are using the IEC or NEC definitions.

Grounding the input

Electrical wires connected to a terminal
Fig. 3: Mains input connection on a Class 1 power supply (RACM600L). Next to the earth connection terminal is the case earthing point, also marked with the ground symbol.
A class I power supply that uses a protective earth connection for both safety and noise filtering is shown in Figure 3:

Figure 4 shows a simplified block diagram of a typical AC/DC class I switching converter. The switching stage generates electrical noise (shown in red) that is conducted both back to the input and across the isolation transformer coupling capacitance to the output. Noise filtering paths (shown in blue) use C2, C3, C5, and C6 to channel the interference to low impedance ground.

The capacitively coupled noise on the output is “shorted” back to the switcher via capacitor C4, thus closing the loop. C4 is placed across the isolation barrier so it must be rated for the isolation withstand voltage (typically 4kV) and be a Y-capacitor type. C1 is a mains-rated X-capacitor and C2, C3, C5, and C6 are all Y-capacitors. For an explanation of the difference between Y- and X-capacitors, refer to the RECOM AC/DC Book of Knowledge, ...

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