Ripple on low voltage

Do you have 35V DC average with 24V p-p superimposed or 24V RMS ?

I'd expect more like 35V RMS, dipping to 0V at the nulls, and so about 1.4* 35v at the peaks.  Be aware that a normal meter with an AC range really measures p-p and then displays 1/2.8 of this - most meters do not perform a true RMS.

There is no 10% rule in BS7671.

I'm not convinced that BS7671 is the correct standard actually, real railways do not follow it, and neither generally do fun fairs, however the physics of what may be a nasty shock do remain, so the while you may argue about the  volt limits (telephones manage 50V DC with 80V AC superimposed for ringing, and rarely do you see dead BT engineers dropping off poles outside when it rains) there is a sensible argument for  minimising the risks as much as possible.

IF (and only if) this is earthed on on side only, the non-earthed pole, if accessible to touch might be dangerous to a  very wet body.

Depending on the era it was originally designed you might actually be centre earthed or indeed centre earthed by two lamps, accross the DC both at half brightness normally, and one bright and one dim during a fault,

IF you decide  do want to smooth it, can you say  how much traction current is typical ?

( 1/10 of a Farad  at 50V is not a lot these days, (ten of these is change from £50. let them cool though and fuse them individually to be sure ripple current is shared and in limits)  and you may not need that much to take the worst off it - and too much capacitance will do the rectifiers and the power factor no good at all.  (I have a rule of thumb that starts  thinking 1000uF per amp, but there is a lot of wiggle room, and an L-C filter may be better for the power factor. )

The old fashioned solution up to is to ballast with a larger version of having three car batteries rather than capacitors but be aware that the design of controls and swtches may rely on the dippy waveform to avoid arc burn, and not appreciate a truly smooth supply..


regards Mike.

Do you have 35V DC average with 24V p-p superimposed or 24V RMS ?

I'd expect more like 35V RMS, dipping to 0V at the nulls, and so about 1.4* 35v at the peaks.  Be aware that a normal meter with an AC range really measures p-p and then displays 1/2.8 of this - most meters do not perform a true RMS.

There is no 10% rule in BS7671.

I'm not convinced that BS7671 is the correct standard actually, real railways do not follow it, and neither generally do fun fairs, however the physics of what may be a nasty shock do remain, so the while you may argue about the  volt limits (telephones manage 50V DC with 80V AC superimposed for ringing, and rarely do you see dead BT engineers dropping off poles outside when it rains) there is a sensible argument for  minimising the risks as much as possible.

IF (and only if) this is earthed on on side only, the non-earthed pole, if accessible to touch might be dangerous to a  very wet body.

Depending on the era it was originally designed you might actually be centre earthed or indeed centre earthed by two lamps, accross the DC both at half brightness normally, and one bright and one dim during a fault,

IF you decide  do want to smooth it, can you say  how much traction current is typical ?

( 1/10 of a Farad  at 50V is not a lot these days, (ten of these is change from £50. let them cool though and fuse them individually to be sure ripple current is shared and in limits)  and you may not need that much to take the worst off it - and too much capacitance will do the rectifiers and the power factor no good at all.  (I have a rule of thumb that starts  thinking 1000uF per amp, but there is a lot of wiggle room, and an L-C filter may be better for the power factor. )

The old fashioned solution up to is to ballast with a larger version of having three car batteries rather than capacitors but be aware that the design of controls and swtches may rely on the dippy waveform to avoid arc burn, and not appreciate a truly smooth supply..


regards Mike.