The International Electrotechnical Commission (IEC) standard IEC 60664: Insulation coordination for equipment within low-voltage systems specifies the requirement for clearances, creepage distances, and solid insulation for equipment based upon their performance criteria.
Motor drive systems typically consist of high-voltage and low-voltage subsystems. Such systems demand reliable galvanic isolation to isolate high-voltage circuits from low-voltage circuits.
To better assimilate the method to calculate the insulation distances when designing the circuit board in motor drive system, this document aims to present the insulation design guide and example to meet the requirement coming from IEC60664-1.
Clearance: Defined as the shortest distance between two conductive materials measured through air. Sufficient clearance distance prevents an ionization of the air gap and a subsequent flashover. Similar to creepage distance the pollution degree, temperature and relative humidity influence the tendency for a breakdown. Breakdown along a clearance path is a fast phenomenon where damage can be caused by a very short duration impulse. Therefore, it is the maximum peak voltage, including transients (overvoltage category [OVC] level), that is to be used to determine the required clearance spacing. Figure 1-1 shows the definition of clearance.
Creepage: Defined as the shortest path between two conductive materials measured along the surface of an isolator which is in between. Maintaining a certain creepage distance addresses the risk of tracking failures over lifetime. The generation of a conductive path along the isolator surface due to the high voltage applied over time is related to the root mean square (RMS) value and depends on environmental conditions, which are described by a pollution degree and the material characteristics of the isolator comparative tracking index (CTI). Breakdown of the creepage distance is a slow phenomenon determined by DC or RMS voltage rather than peak events or transients. Inadequate creepage spacings can last for days, weeks or even months before failure. Figure 1-2 shows the definition of creepage.
Functional insulation: Insulation between conductive parts which is necessary only for the proper functioning of the equipment. Functional insulation passes a signal or power from a system at one voltage potential to another system and a different voltage. Functional insulation does not protect against electrical shock..
Basic insulation: Insulation applied to live parts to provide basic protection of persons against electric shock.
Double insulation: Double isolation takes a system with basic insulation and adds a supplementary insulation layer between the electrical parts and the end user to reduce the likelihood of electrical shock in the event that basic isolation fails.
Reinforced insulation: A single insulation system applied to live-parts, which provides a degree of protection against electric shock equivalent to double insulation. A device with reinforced isolation provides basic isolation; plus, the insulation is designed to provide the physical separation between printed circuit board traces, cores, windings, pins, and so forth.
DVC: Decisive voltage class is the classification of voltage range used to determine the protective measures against electric shock. The limit of DVC can be referred to table 3 of the international standard IEC61800-5-1 (adjustable speed electrical power drive systems, part 5-1: safety requirements for electrical, thermal and energy)
ELV: Extra low voltage defines any voltage not exceed 50V ac-rms and 120V dc. Protection against electric shock is dependent of the DVC. DVC A and B are contained in the voltage range of ELV.