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Ripple on low voltage

I look after a small fairground type railway. It is powered via a transformer and full wave rectifier and runs at around 35 volts. At recent inspections it has been noted that it fails as there is around 24 volt ripple on the 35 volt dc, which is not surprising as there is no smoothing. Apparently according to the person undertaking the inspection, who states it is to bs7671, the ripple should be no more than 10% ripple, however I cannot find any such requirement.

Has anyone got any experience of this type of thing, I am loathe to add a capacitor to smooth the dc - it would have to be probably around 0.1F, and ripple current in it could well be a problem.

Any thoughts on this would be appreciated, is there an actual requirement on ripple, or is it OK to just have full wave rectified dc as long as the peak voltage does not exceed the elv limits of 120v dc or 50 v ac rms .

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0 gkenyon over 4 years ago

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?
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0 gkenyon over 4 years ago

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?
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Ripple on low voltage

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