Note 1 to entry: The transfer function of a second-order lag element is given as

${\displaystyle \frac{V\left(s\right)}{U\left(s\right)}=\frac{K_{\text{Ρ}}}{\left(1+T_1\cdot s\right)\cdot \left(1+T_2\cdot s\right)}}$ for two real poles

and as

${\displaystyle \frac{V\left(s\right)}{U\left(s\right)}=\frac{K_{\text{Ρ}}}{\frac{s^2}{{\omega }_0^2}+2\cdot \vartheta \cdot \frac{s}{{\omega }_0}+1}}$ for a complex conjugate pair of poles

where

KP	is the proportional action coefficient;
T1, T2	are the time constants;
$\vartheta$	is the damping ratio;
ω0	is the characteristic angular frequency;
s	is the complex variable of the Laplace transform;
U(s)	is the input transform;
V(s)	is the output transform.

Note 2 to entry: This entry was numbered 351-28-14 in IEC 60050-351:2006.