Effective collision strengths Υ at temperatures of T_e = 5 × 10²(z + 1)², 10³, 10⁴ K (here, z = Z − 3) along the iso-electronic sequence. Left-hand panels: 1s2s2p   ⁴P_3/2  →  1s²2s   ²S_1/2 (u line), 1s2s2p   ²P_1/2  →  1s²2s   ²S_1/2 (r line) and 1s2s2p   ⁴P_1/2  →  1s²2p   ²P_3/2 (3-26) transitions. Right-hand panels: 1s2p^2   4P_1/2  →  1s²2p   ²P_3/2 (h line), 1s2p^2   2D_3/2  →  1s²2p   ²P_1/2 (k line) and 1s2s^2   2S_1/2  →  1s²2p   ²P_3/2 (3-25) transitions. [Colour online]