Bob,
I have added my view of what the requirements are of IEC 950, 2nd ed, and A3.
Brought to you by Johnson's Vax wrote:
>
> 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
> ----------
Based on your assumptions I see the following being required:
CIRCUIT TRANS. CLEARANCE OPERAT. BASIC REINF.INS.
mains 120 (= 150) Vac 1,5 kV Tables 3 + 4 0,7 mm 1,7 mm 2,4 mm
mains 230 (= 300) Vac 2,5 kV Table 3 1,4 mm 2,0 mm 4,0 mm
TNV network (150) 1,5 kV Table 5 0,7 mm 1,0 mm 2,0 mm
TNV network (300) 1,5 kV Table 5 1,6 mm 1,9 mm 3,8 mm
A TNV circuit is a secondary circuit (1.2.8.8), Table 3 applies for primary to
secondary, and Table 5 for TNV to TNV.
Note: 2.2.6 requires only operational insulation between TNV and another TNV
circuit.
The 300 Vpp ringing signal is above the 120 V mains peak voltage, therefore,
additional 0,2 mm need to be considered here, but is below the 230 V mains peak
voltage.
> 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
>
When using the interpolation option (Condition 7 to Table 6), the spacings may
be
slightly reduced as follows:
Mains to TNV:
mains 120 Vac 230 Vac
Basic insulation 1,8 mm 2,7 mm
Reinforced insulation 3,6 mm 5,4 mm
TNV to TNV:
Basic insulation 2,4 mm
Reinforced insulation 4,8 mm
--
Kind regards, Horst Dierich, Germany
EMAIL: [email protected]
