Creepage vs Clearance in Electric Motors: Difference and Drawing Dimensions

Creepage and clearance differ in their path, in the voltage they are sized for and in the way they fail. Clearance measures the direct path through air, creepage distance the path along the surface of an insulating material. Geometrically, the creepage distance is therefore never shorter than the clearance between the same two parts. Every edge, fold and overhang the surface path has to go around makes it longer.

Clearance is sized for the voltage peaks that can occur, that is impulse voltages and transient overvoltages. If it is too short, the air breaks down instantly. Creepage distance is sized for the RMS value of the voltage that is permanently applied. It does not fail suddenly but over time: moisture and deposits allow leakage currents to flow, which damage the surface step by step until a conductive path forms.

The required clearance depends mainly on the impulse voltage it has to withstand. Pollution degree, the shape of the electric field and altitude also play a part. The dielectric strength of air falls as air density falls, so clearances have to be increased for installation altitudes above 2,000 m.

The required creepage distance depends on three factors: the working voltage, the pollution degree and the material. The pollution degree describes in four levels what to expect at the installation site, from no pollution or only dry, non-conductive pollution (degree 1) to persistent conductive pollution (degree 4). How well an insulating material resists tracking is described by its tracking resistance, given on the data sheet as the CTI (Comparative Tracking Index). On this basis, insulating materials are classified into material groups, from group I with a CTI of 600 or more down to group IIIb. The more tracking-resistant the material, the shorter the creepage distance may be under otherwise equal conditions.

The basics are set out in the standard on insulation coordination for equipment within low-voltage supply systems, IEC 60664-1 (EN 60664-1 in Europe). It covers equipment up to 1,000 V AC or 1,500 V DC, and high-voltage machines follow other rules. The specific values for a machine are set by the product standards that apply to it. For motors in hazardous areas with increased safety protection, for example, IEC 60079-7 sets its own minimum values, and for motors in household appliances IEC 60335-1 does.

A key creepage distance in the stator lies at the slot end, where the winding leaves the lamination stack. If the slot liner protrudes beyond the stack there, the surface path from the winding to the core has to go around this overhang. The longer the overhang, the longer the creepage distance. The cuff, a folded edge on the slot liner, works in the same direction: its geometry helps maintain the required creepage distance, and it keeps the liner in place during winding.

The thickness of the slot liner, by contrast, adds little to the creepage distance. It determines the dielectric strength through the material and competes there with the copper share in the slot, as explained in how thin a slot liner can be.

Between two winding layers in the same slot, the slot separator provides the separation. It is often longer than the lamination stack and protrudes at the ends, so the separation extends beyond the slot. More on this under when a slot separator is needed. In the end winding, the phases pass close to each other outside the slot. Flat die-cut parts matched to the end winding sit between them, see fitting and supplying phase separators. Which part sits where in the stator is shown in the overview of all stator winding insulation parts.

Creepage and clearance are set by the manufacturer of the motor or generator during design, because only the machine builder knows the voltage, insulation concept, operating environment and applicable standards. The result is not a set of distances on paper but dimensions: length and overhang of the slot liner, length of the slot separator, dimensions of slot closure and phase separator. These dimensions are on the technical drawing, and the drawing is the document every delivery can be measured against. Our part is to produce length, overhang and cuff exactly as they appear there.

A drawing that can be produced from without further questions starts with the geometry: length, width or developed width, profile and leg lengths, for slot liners the slot geometry including any skew, and for a slot closure the back width and leg length.

Next come the dimensions that carry your creepage and clearance: the slot liner overhang at both ends, the length of slot separator and slot closure, design and size of the cuff, and the contour of the phase separators. State the stack length as well, so every overhang has a clear reference.

Add tolerances, above all for every dimension that governs a distance, and the material with grade and thickness, for example NPN, DMD or Nomex® 410. If tracking resistance is part of your design, specify the material itself and not just the thermal class, because two materials of the same thermal class can differ in CTI. Finally, quantities and supply form: lengths, rolls or finished die-cut and formed parts.

A creepage distance that is correct on the drawing only helps if the part holds the dimensions. Overhangs, cuff and lengths that are clearly dimensioned and toleranced give you insulation parts that can be checked against the drawing with every delivery. The insulation parts we make for motors are listed on the electric motor manufacturing page.