Ripple on low voltage

BS 7671 requires the DC to be ripple-free, otherwise it's classed AC. BS 7671 no-longer talks about "ripple-free" being 10 % - however, it's in BS IEC 60479-1 (Note 1 to Clause 6.1), and therefore I think the inspector is correct - if it's got more than 10 % ripple, then for the purposes of protection against electric shock it becomes AC and not DC.


Hence, this system would strictly be an ELV system operating at 35 V AC peak.


There is no requirement for extra-low voltage to be DC - BUT, I would caution that the 120 V DC / 50 V AC limit for SELV and PELV is for dry condition only.


Now the issue with 35 V AC is that normally, when we talk about "wet but not submerged" conditions, BS 7671 (and other standards such as BS EN 60204-1) limit the voltage to 25 V AC / 60 V DC.


35 V AC rms would be too much for conditions that may not be "dry".

EDITED - to add the following:


However, this 35 V AC is "peak" - when we talk about AC, we talk about the rms value - and therefore it's highly likely that we are within the range of 25 V AC rms (35.3 V peak full-wave rectified) and it should therefore be in the acceptable range?

Is a 3 phase supply available ?

If so, then rectified three phase will produce much less ripple than rectified single phase.

Other options include a large switched mode power supply with a smoothed and regulated output. 

Power supplies for electric railways, even miniature ones, need careful design. A standard SMPSU can be killed by an external voltage applied to its output. Fit a large silicon power diode between the SMPSU, to prevent any back fed voltage if the supply fails and the motor on the train acts as a generator. Also consider a resistance of say 0.1 to 0.25 ohms that is shunted across the  DC supply by a contactor when the mains fails. This will prevent any undesireable voltage being produced. Under normal conditions this resistor will disipate nothing, but it must be able to handle a few Kw for few seconds, a calculated length of cable will serve.

Presuming a short line and modest speeds, consider on board battery power, a relatively small battery would suffice. I have seen a miniature electric railway powered by a 24 volt nominal battery of about 300 AH. Charge overnight. Fit an interlock to stop the train being driven whilst plugged in to the charger.


How touchable is the conductor rail ? if protected against easy contact you MIGHT be able to justify the present arrangements.

I think that my first question would be what instruments were used to measure the dc and ripple?  The waveform would be non-sinusoidal so could not be measured with average reading rms indicating instruments.  The peak voltage could be estimated by measuring the ac input to the rectifier and calculating.


By my calculation 35 Vdc from a full wave rectifier would have a peak voltage just under 55 V.


David

Another thought, is the output referenced to earth or floating?


David

My money is on their being a failed diode.

I am assuming that there is just a simple secondary fed to a bridge rectifier, ie 4 diodes.  Could be a centre-tapped transformer, with a single diode from each end with the centre-tap either positive or negative depending which way round the diodes have been fitted. In either case, the loss of a diode will give you half-wave rectification.

Clive

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.

IME, most miniature electric railways use either a live third rail for the positive with the earthed running rails for the negative return (like a miniature version of the Southern region main line electric railways) This system has the merit that the metalwork of the train is fairly reliably close to earth voltage and safe to touch by passerngers boarding/alighting.


Or alternatively they have the positive on one running rail and the negative on the other rail (like an enlarged model railway) This system requires special rolling stock with the wheels insulated from each other.  It is potentialy less safe since nothing prevents one side of the supply becoming earthed by leakage. If a fault on the train results in the metalwork becoming connected to the now live side of the supply, then anyone boarding/alighting is subjected to the full supply voltage between hands touching the train, and feet touching the ground. This can be dangerous even at very low voltages. Consider a sudden rain storm on a hot day when persons might go unshod. The risks may be much reduced if the train is largely of wooden construction, with metal door handles and the like mounted on timber rather than metal.


As an aside, the still extant Volks electric railway in Brighton originally used a nominal 50 volts DC supply between the running rails, but was later changed to a live rail with return via the earthed running rails. AFAIK it still uses 50 volts DC and the original rolling stock. It claims to be the oldest electric railway in the world.

Interesting, I am  happy to be corrected, I admit was expecting something more like the 4 track DC railways of the London underground and the Isle of Wight - though the main safety in all of those is the fact that the rails are only energised in sections when the train needs  it - at that point being electrocuted is a far lesser risk than being run over, and the accident figures bear that out.,

I can imagine on a short run the sectional energising idea  is largely  irrelevant. I am aware of things like dodgems being supplied at  90- 100V DC with an earthed electric floor, and 'out of reach' being the normal safety measure, the contact  spoon on the car, and the mesh being too high for the tallest user.

Assuming then it is earthed, the killer  question is how real is the risk of the power rails  being touched while live, with enough contact area to skin to be a hazard - I;m not convinced that smoothing is really needed or desirable.

M.

At the risk of going O/T I feel that you are mistaken regarding the conductor rails on London Underground and other full size electric railways. The conductor rails are live continually and only isolated for maintenance work or in an emergency.

The main safety precuation is to exclude unauthorised persons.

Thanks to you all who have replied, your input is very helpful and much appreciated. To put the record straight, it is a very simple 2 rail affair, in essence the same as a model railway, but much bigger with the supply to the train via the wheels, with those on one side having insulated bushes and those on  the other side being connected to the engine metalwork. I don't think that either rail is actually earthed, but the rails are not insulated as such from the earth. I hope to be on site next week and will take a scope along to measure the actual voltage.  Sadly 3 phase not available and whilst on-board battery is a good idea, speed is controlled by voltage applied to track and polarity is reversed when train stationary to provide power to lights and therefore a complete redesign would be needed to use that approach. Max current is around 30 amps, but will try and measure this next week.