An air gap increases the reluctance of a CT magnetic circuit. It can reduce retained flux and shorten the magnetic response time, but it also increases the excitation required for a given flux. For transient-duty cores, that tradeoff must be related to the fault duty and the relay’s measurement needs. [1] [2]
ABB’s application guide discusses low-remanence and larger-gap cores separately. It also warns that gap width must remain constant. A distributed-gap arrangement adds a practical issue: the total gap can look correct while individual gaps move or become unequal. [1]
Compare total reluctance and local geometry
Ignoring fringing, the air-gap contribution is Rg ≈ Σ[gj/(μ0 Aj)]. Gap gj is in metres, effective gap area Aj is in square metres, μ0 is the permeability of free space in H/m, and reluctance Rg is in H⁻¹. Include the steel path separately and recognize its nonlinear magnetic response.
With equal area, equal total gap length gives a similar first estimate of gap reluctance. It does not prove the same fringing field, winding exposure to leakage flux or mechanical stability. Distribution changes where those effects occur.
| Gap observation | Magnetic implication to investigate | Mechanical evidence |
|---|---|---|
| One gap becomes smaller | Lower total reluctance and altered residual-flux response | Spacer compression or joint movement |
| Unequal gaps with unchanged sum | Similar first-order reluctance but different local fringing | Individual gap positions and support state |
| A gap becomes bridged | Unintended magnetic path or local contact | Spacer identity and foreign-material inspection |
| All gaps retained after assembly | Geometry available for intended response | Measurements tied to the assembled core |
Evaluate reset against the next fault
The relevant residual flux is the flux remaining under a defined magnetic history and elapsed time. Relate it to the interval before a repeated fault, rather than assuming that low remanence means instantaneous reset. Secondary winding resistance and the connected burden influence the loop’s time constant along with magnetic inductance.
A larger gap can reduce the CT’s reproduction of the decaying DC component. That may be acceptable for one specified transient class and unsuitable for another measurement requirement. Avoid calling the design better solely because the remanence is lower.
Preserve the gaps through manufacturing
Assign identifiers to the individual gap locations and retain their intended spacer material, dimensions and restraints. Compare core measurements before winding with the final assembled condition; winding tension and clamping can change a joint that was correct when bare.
Use catalogue 0.23 mm gauge [3] with assumed 200 net layers and 20 mm width: A = 920 mm². Four 0.05 mm gaps or one 0.20 mm gap give Rg = 172,995 H⁻¹, neglecting fringing. One 0.02 mm closure lowers total gap to 0.18 mm and Rg by 10%. In an additional gap-dominated linear screen with assumed N = 500 and total secondary-loop resistance 1 Ω, L = N²/Rg and τ = L/R are 1.445/1.606 s [1]. After 1 s, initial flux-linkage fractions e⁻ᵗ/τ are 50.06%/53.64%. An assumed 52% transient-reset screen passes nominal and fails closed-gap. This is circuit decay, not ABB’s three-minute material-remanence measurement. Inspect individual final gaps and validate actual nonlinear transient duty before release.
Individual spacer thickness, seating, contact cleanliness and clamp load preserve the distributed arrangement. Keep each gap’s identity and final state on the release record instead of recording only a summed gap length.
The acceptance evidence should connect three things: stable gap geometry, excitation characteristics and the defined transient duty including any repeated energization. A calculation of total gap reluctance is useful for screening changes. It cannot replace evidence that the installed gap distribution still produces the required magnetic response after winding, handling and clamping.
References
- ABB, Instrument Transformers Application Guide, publication 1HSM 9543 40-00en, Edition 4, February 2015, sections 3.4 and 4.9–4.12.
- IEC, IEC 61869-2:2012, Instrument transformers — Part 2: Additional requirements for current transformers.
- Nippon Steel Corporation, Grain-Oriented Electrical Steel Sheets, catalogue D004je, April 2020, specification and typical-property tables.