Lesson 3 of 7

Insulation Coordination & Withstand Levels (BIL)

Every piece of high voltage equipment has to survive both its everyday operating voltage and the occasional, much larger voltage spike from lightning or switching. Insulation coordination is the discipline of deliberately choosing how strong each piece of equipment's insulation is — and how it relates to the protection around it — so the whole system fails safely and predictably if it fails at all.

The core idea

You could make every piece of equipment massively over-insulated to survive any conceivable overvoltage, but that's expensive, bulky, and eventually physically impractical. Insulation coordination instead sets a deliberate, economically justified insulation strength for each piece of equipment, and pairs it with protective devices (surge arresters, covered in the next lesson) that are guaranteed to limit any overvoltage below that equipment's withstand level before it's ever tested for real.

Basic Insulation Level (BIL)

BIL is the rating most engineers know for how strong a piece of equipment's insulation is against lightning overvoltages. It's expressed as the peak voltage of a standardized lightning impulse waveform (in practice, a voltage pulse that rises to its peak in about 1.2 microseconds and decays to half its value by about 50 microseconds) that the equipment is proven able to withstand.

Why a standardized waveform

Real lightning strikes and switching events produce all sorts of different transient shapes. Standardizing on one reference waveform (defined by international standards such as IEC 60060) means a BIL rating from one manufacturer is directly comparable to another's — without it, every piece of equipment would need its own bespoke, incomparable test.

BIL vs IEC terminology

“BIL” (and its switching-surge counterpart, BSL) is ANSI/IEEE terminology, defined in IEEE Std C62.82.1 and used throughout North American equipment ratings. IEC 60071 — the standard applied in Europe and most of the world — does not use the term BIL. It specifies standard rated withstand voltages instead: a rated lightning impulse withstand voltage for all equipment, plus a rated switching impulse withstand voltage above 245 kV (Range II) or a rated short-duration power-frequency withstand voltage up to 245 kV (Range I). The physical idea is the same — a proven peak value of a standardized impulse — but on an IEC datasheet you'll find it as, for example, “U_p = 1050 kV” rather than “BIL 1050 kV”.

How coordination actually works

System (operating) voltage: The everyday nominal voltage the equipment operates at continuously — the starting point insulation is designed above, not the limit it's tested against.
Representative overvoltages: The transient voltages the system is expected to experience — from lightning strikes, switching operations, or temporary faults — estimated through system studies.
Protective level: The maximum voltage a surge arrester will let through during an overvoltage event, once it has done its job of clamping the surge.
Withstand level (BIL / rated withstand voltage): The equipment's rated insulation strength, set with a safety margin above the protective level — so the arrester is always guaranteed to act first.

The whole exercise only works if this ordering holds: protective level clearly below withstand level, with enough margin to account for manufacturing tolerances, aging, and installation variation. Get that margin wrong, and either the equipment is under-protected (arrester doesn't clamp low enough) or it's needlessly over-insulated (wasted cost).

Higher voltage class ≠ proportionally higher BIL

As nominal system voltage increases, required BIL increases too, but not in simple direct proportion — very high voltage (EHV) systems benefit from better-controlled switching surges and more sophisticated overvoltage limitation, so BIL as a fraction of operating voltage generally decreases at higher voltage classes. This is one of the reasons insulation coordination is treated as its own engineering study for every voltage class and installation, not a simple lookup formula.

Key takeaways

Insulation coordination balances equipment insulation strength against protective devices, rather than over-insulating everything.
BIL (Basic Insulation Level) is the ANSI/IEEE name for a standardized lightning-impulse withstand rating; IEC 60071 expresses the same idea as a rated lightning impulse withstand voltage.
Coordination requires the surge arrester's protective level to sit safely below the equipment's withstand level, with margin.
The required margin and relative BIL level are determined by system studies, not a single fixed rule across all voltage classes.