The shortest distance between two conductors through air in between is called the Clearance. It depends on the factors mentioned below.
- Peak Working voltage across insulation (Line-Neutral)
- Transient Overvoltages that may occasionally appear on the line conductors.
- Short term & long term Overvoltages that may occur between the line conductor and earth in electrical installations.
- Altitude
Creepage: This is the shortest distance between two conductors along the insulating surface in between. It depends on the above-mentioned factors of clearance as well as some other factors mentioned below.
- Material group (l/11/llla/lllb)
- Pollution degree
- R.M.S. working voltage
Note: Clearance and creepage distance shall be within the values mentioned in different Safety Standards like I EC 60950-1/1 EC 61010-1/1 EC 62368…etc)
Behaviour of Creepage & Clearance under High Stress
At high frequencies or in large clearances, the ions are trapped between the electrodes, since the polarity changes too rapidly for them to be extracted at either end of conductors on PCB.
At high frequencies or in large clearances, the ions are trapped between the electrodes, since the polarity changes too rapidly for them to be extracted at either end.
Unlike clearances, creepage distances and solid insulation are not replenishable because permanent damage, such as carbonized tracking and puncture, are likely.
Clearances are the leading cause of the phenomenon observed. Since charged ions travel along the electric field lines, any change in field direction could cause the ions to collide with, or be reflected away from, the dielectric surface. PD occurs in the clearances along the surface where the ionized gas is trapped, deteriorating carbon-based surface material, and making it more susceptible to carbonized tracking.
Determination of Clearances, Creepage Distances with High Frequency Voltage Stress
Insulation coordination is a discipline that seeks to achieve the optimal technical and economical balance necessary to protect people and equipment against overvoltages, with the goal of enhancing system operability (e.g., continuity of the electrical supply) and product safety.
It is implemented by matching the electric strength of equipment with respect to surrounding environmental conditions and the electrical stresses that are expected to occur throughout its anticipated lifetime, such as:
- Repetitive working voltages necessary for the intended functioning of the equipment
- Transient overvoltages of atmospheric origin or switching surges
- Temporary overvoltages due to earth fault in the power distribution system, from which the A.C. mains supply is derived
- Transient overvoltages due to interconnection to other equipment, or connection to telecommunication networks or cable distribution systems
Creepage Distances: Though the dimensioning of creepage distances is of little relevance to random transient overvoltages, it is commonly understood that the creepage distances must be equal to or greater than the associated clearances.
In summary, for frequencies not exceeding 30kHz, only the RMS-value of any periodic voltage is relevant. For frequencies greater than 30 kHz, the dielectric breakdown resulting from the effects of PD has priority over the phenomenon of carbonized tracking, due to the accumulation of charges along the insulating surface.
Dimensioning creepage distances can be ignored if the surface material is not likely to be deteriorated by thermal effects (e.g., ceramics). However, dimensioning clearances still applies in such cases.
Case Study: Investigation of a Switch Mode Power Supply
In this section, determining appropriate clearances, creepage distances and solid insulation will be illustrated through the investigation of a novel Switch Mode Power Supply (SMPS), designed for compliance with various energy saving standards.
In general, the choice of operating frequency is achieved by balancing the following factors:
- Acoustic Noise: Ears can normally hear sounds in the range of 20 Hz to 20kHz; hence, the operating frequency would not fall into this range (usually above 20kHz).
- Power Density: In theory, power density is proportional to the operating frequency; the higher the operating frequency, the smaller form factor.
- Efficiency: Higher operating frequency comes with higher switching loss and the associated conduction loss.
- Components: The tradeoff between high performance and cost-effectiveness.
- EMC: Challenges associated with electromagnetic compatibility issues.
The operating frequency of a modern SMPS often ranges from 20kHz to 1MHz. (For example, a high power inverter with IGBTs, SMPS with Power MOSFETs, and DC-DC converters with MOSFETs would operate in a range of 20-30kHz, 50-200kHz and 100-400 kHz respectively.) The application of frequencies above 1MHz is rarely seen in practice, and a steep slope (i.e., a rapid change in behaviour) is usually observed when the frequency is above 1MHz.
The investigation discussed here was conducted with an SMPS employing a variety of devices, including a PFC controller with standby mode programming, a half-bridge LLC resonant converter with adjustable operating frequency up to 500kHz tailored for a broad range of load conditions, a Synchronous Rectifier (SR) controller, and a synchronous step-down switching regulator.
General technical information regarding the SMPS is as follows
- Rated voltage range: 100-240 V a.c., 50/60 Hz
- Overvoltage category II: Pluggable equipment supplied by fixed electrical installation of buildings
- Pollution degree 2
- Peak working voltage: 514 V peak
- RMS working voltage: 270 V RMS
- Short-term temporary overvoltages: 1,440 V RMS, if it is intended to be supplied by IT or TI power system
- Operating frequency: 500kHz
Comparative tracking index (CTI): Material Group Ill
Clearances
Clearances to withstand transient overvoltages
Rated Impulse Voltage referenced from IEC 60664-1: Nominal system voltage<::; 300 V A.C.
Overvoltage category: II
Rated impulse voltage for equipment: 2,500 V
Clearances for Case A (inhomogeneous field) from Table F.2 of IEC 60664-1: Required impulse withstand voltage: II
Pollution degree: 2
Clearances for basic insulation= 1.5 mm
Clearances for reinforced insulation =3.0 mm
Clearances to withstand periodic voltages (taken into account for high frequency voltage stress)
Clearances for Case A (inhomogeneous field) from Table 1 of IEC 60664-4, upon the condition that the operating frequency is above f (critical):
Peak working voltage<::; 600 V Peak Clearances for basic insulation= 0.065 mm Clearances for reinforced insulation = 0.50 mm
Creepage Distances
Creepage distances to avoid carbonized tracking Creepage distances referenced from IEC 60664-1: RMS working voltage: 270 V r.m.s.
Pollution degree: 2
Comparative tracking index (CTI): Material Group Ill Creepage distances for basic insulation= 2.7 mm Creepage distances for reinforced insulation= 5.4 mm
- Creepage distances to avoid deterioration due to high frequency voltage stresses
Creepage distances from IEC 60664-4: Peak working voltage: 514 V peak
Pollution degree: 2
Operating frequency::<; 500kHz
Creepage distances for basic insulation = 0.6 mm
Creepage distances for reinforced insulation = 1.2 mm
For creepage distances, the effect of carbonized tracking takes priority over the effect of deterioration due to high frequency voltage stresses. Therefore, the minimum creepage distance for basic insulation is 2.7 mm, and that of reinforced insulation is 5.4 mm. No alignment is needed as the minimum creepage distances are greater than the minimum clearances (Creepage of Reinforced insulation= 2* Basic Insulation)
A comparison between different product safety standards
Instead of a comprehensive comparison of the dimensioning of clearances, creepage distances and solid insulation found in IEC 62368-1and the legacy standards (i.e., IEC 60950-1and IEC 60065), this section offers a brief overview through a series of simplified charts.
In brief, the clearances of the legacy standards are obtained by assuming the A.C. mains supply does not exceed 300 V RMS (i.e., A.C. mains transient of 2,500 V peak, representing equipment designed for a universal input voltage range of 100-240 V A.C.), pollution degree 2, overvoltage category II. Hence, the traces of the legacy standards depicted result in an offset of 2 mm for basic or supplementary insulation, and 4 mm for reinforced insulation, and associated additional values in Table 2L of IEC 60950-1.
The first crossover of legacy standards and creepage distances for fundamental frequencies exceeding 700kHz but equal to or less than 1,000 kHz (Derived from IEC 60664-1) occurs at 600 V peak. This means that, if peak working voltages exceeding 600 V peak and operating frequencies exceeding 700kHz are likely, a rapid demand growth for creepage distances is inevitable.
Conclusion
Emerging energy saving standards have Ied to a marked preference for switch mode power supply topologies. However, the use of SMPSs has raised concerns over the sharp change in dielectric properties under high frequency voltage stress, and the attendant safety risks. This article has provided some basic information about the partial discharge phenomena, and the rationale underlying the applicable product safety standards, in an effort to assist compliance engineers to better understand applicable testing requirements.
Nirod Ranjan Samantaray is a Senior ReguIatory Compiiance Specialist.
