Loss, temperature rise and core flux — engineering reference

Short-circuit apparent power of the upstream network at the connection point, Ssc = √3·Un·I″k, or the equivalent source impedance behind it.

Why it matters: It is the single input that says how "stiff" the grid is. Without it no fault-current figure can be computed, so it must be requested from the utility (or taken from a stated assumption) before any switchgear can be quoted.

Ratio of the equivalent reactance to the equivalent resistance seen from the fault point (IEC 60909-0).

Why it matters: It fixes two things at once: the peak factor κ (hence ip) and the decay rate of the DC component. A high X/R (inductive LV feeds, generator sources) gives both a higher peak and a longer DC transient, which the breaker must interrupt.

Peak factor κ = ip / (√2·I″k): the ratio of the first peak to the rms value, calculated in IEC 60909-0 from the R/X ratio of the equivalent impedance.

Why it matters: It is what converts a short-circuit current into a mechanical force. An inductive network (LV cables, transformers) pushes κ towards 2.0 and raises the peak force, which is why ip cannot be derived from Ik alone.

Peak short-circuit current ip = κ·√2·I″k (IEC 60909-0): the highest instantaneous value of the fault current, reached about half a cycle after inception.

Why it matters: It is the number that generates the electrodynamic force on busbars, cable cleats and device terminals. Compare it with the rated peak withstand Ipk — quoting only Ik leaves the mechanical strength of the switchboard unchecked.

No-load loss of a transformer: the power absorbed at rated voltage and rated frequency with the secondary winding open (IEC 60076-1). It is essentially the core (hysteresis + eddy) loss.

Why it matters: It runs continuously — about 8 760 h a year — so it dominates the lifetime energy bill. Tenders commonly capitalise it at a $/W figure, which means a lower P0 can justify a higher purchase price.

Load loss (short-circuit loss) of a transformer: the power absorbed at rated current with the secondary short-circuited, corrected to the reference temperature (IEC 60076-1). It includes the I²R loss plus stray and winding eddy losses.

Why it matters: It grows with the square of the load and is the basis of efficiency guarantees and of any loss-capitalisation clause. Pk and uk% come from the same test, so a datasheet giving one without the other is incomplete.

Short-circuit impedance voltage of a transformer: the primary voltage, in percent of rated voltage, that drives rated current through the short-circuited secondary winding (IEC 60076-1).

Why it matters: It fixes the LV fault level (approximately I″k ≈ In / uk) and at the same time the voltage drop under load. This is a real quotation trade-off: a larger uk lowers the fault level (cheaper switchgear downstream) but increases voltage drop and losses. Always quote uk together with the transformer rating.

Highest voltage for equipment Um: the maximum rms line-to-line voltage at which the equipment may be operated (IEC 60071-1).

Why it matters: Um anchors the whole insulation-coordination table: it decides which lightning-impulse and power-frequency withstand values are required. Buying equipment with too low an Um is not a saving, it is a non-conforming offer.

Lightning impulse withstand voltage: the peak of the standard 1.2/50 µs impulse wave that the insulation must withstand without breakdown (IEC 60071-1).

Why it matters: It is a named specification and type-test item for switchgear, transformers and bushings. It is one of the first parameters a cheap offer silently reduces, so it must appear explicitly in the datasheet you compare.

Maximum loadability factor λmax of the continuation power flow: the multiplier applied to all loads at the nose point of the PV curve, the point where the voltage collapses.

Why it matters: It is the loading margin of the design: λmax = 1.35 means the network only collapses at 135 % of today load. Planners require a minimum margin (the number differs between grid codes — state the criterion used), and it decides whether a line reinforcement or more reactive compensation has to be quoted.

Loading: the current (or power) carried by a branch expressed as a percentage of its rated capacity.

Why it matters: It is the acceptance number for cables and transformers. Above 100 % the element will trip or have to be derated, and the margin left at the design case is exactly what the customer is paying for — so it must be shown with the assumption (load factor, ambient temperature) that produced it.