I worked out what looks like the standards requirements in IEC 950 for
telecom spacings. Do you see any obvious errors in what I did?
The result is spacings which seem excessive to me.
Bob Johnson
DETERMINATION OF SPACINGS IN TELECOM CIRCUITS
7 from 6.2.1.1b, the maximum signal voltages may be up to 70.7 Vac or 120 Vdc.
7 from M.3.1.2, the maximum ringing voltage may be 300 Vpp or 200 Vp-to-earth
(M.2 for European ringing specifies only currents and cannot be used to
derive operating voltages.)
7 2.2.7.1 requires the maximum voltage difference be considered.
(2.2.7.2 allows SELV voltages to be consided zero with respect to primary,
but not TNV)
7 2.2.7.4 states ringing signals can be ignored for creepage distances
(but not for clearances)
Assuming pollution degree 2, overvoltage category II, material group III,
without the routine testing option.
CLEARANCES
----------
Mains to TNV:
peak difference
120Vac = 170Vp to earth, 230Vac = 325Vp to earth
ringing signal is 200Vp to earth
170 Vp + 200 Vp = 370 Vp
325 Vp + 200 Vp = 525 Vp
these are normal repetitive peak voltages (requiring table 4)
mains 120 Vac 230 Vac
Table 3 2.0 mm 3.2 mm
Table 4 0.2 mm 0.2 mm
Basic insulation 2.2 mm 3.4 mm
Reinforced insulation 4.4 mm 6.8 mm
TNV to TNV
peak difference between two (independent) ringing signals
(respect to earth) is +200 Vp to -200 Vp = 400 Vp;
from table 5:
Basic insulation 1.9 mm
Reinforced insulation 3.8 mm
CREEPAGE
--------
Mains to TNV:
rms difference 120 Vrms + 120 Vdc = 170 Vrms
230 Vrms + 120 Vdc = 260 Vrms
from table 6:
mains 120 Vac 230 Vac
Basic insulation 2.0 mm 3.2 mm
Reinforced insulation 4.0 mm 6.4 mm
TNV to TNV
dc difference +120 Vdc to -120 Vdc = 240 V dc
Basic insulation 2.5 mm
Reinforced insulation 5.0 mm
