Yellow bucket Step-Down transformers, are they actually safe and legal?

KC said:

The centre tapping only really helps protect against line to earth faults.

Yes ... that's the "protection against electric shock" bit, the theory with the reduced low voltage (RLV) single-phase transformer system being that you can only get a 55 V shock ... and the primary side overcurrent protective device can operate to achieve the 5 s disconnection time required by Regulation 411.8.3 of BS 7671.

With respect to "overcurrent" ... a combination of the primary side overucrrent protective device, and thermal trip, provide for that.

As an example of how ADS might work with a 20 A circuit-breaker on the primary.

A B20 requires Zs to be no greater than 2.19 Ohms at 230 V, which is equivalent to 9.5 mΩ p.u. (per unit). If this is reflected to the secondary at 55 V to Earth, this give 55×9.5÷1000 Ω = 0.52 Ω (which is the value in Table 41.6).

Zs for a 13 A fuse is a similar value of just over 2 Ohms at 230 V.

And, because the primary is supplied line to neutral, you will actually have an effective Zs of even less, assuming you're plugging directly into a socket-outlet in the installation. It also means it's usually OK to plug the RLV transformer into a TT system or IT system ...

To meet voltage drop at an outlet for a supply circuit operating at 20 A, so if my "finger in the air" mathematics is working properly, this is roughly equivalent to 2.5 mΩ p.u. maximum, meaning you are likely to meet disconnection times for 13 a fuse or B20 breaker, even if you use extension leads. Assuming a line to neutral supply impedance of 0.35 ohms (don't forget this is line to neutral not line to earth) this is equivalent to 1.5 mΩ p.u., so in total this gives an overhead of 5.5 mΩ p.u.for the transformer internal impedance line to Earth (not line to line), plus R1+R2 of any extension leads used (this totals to about 0.3 Ohms at 55 V).

(PS - above very quick "finger in the air" or "back of a packet" calculations, no guarantees on accuracy, and apologies if I've missed some assumptions. Please check assumptions and working yourself if reading this e-mail.)

There is also the saving grace, that once you have a fault the output voltage tends to droop, so the shock voltage to ground may well be such that there is no need to to disconnect in any specific time - below 50V this is usually considered the case, though personally I would design for less than that - perhaps 25V, in locations where the body may be compromised electrically - working inside a metal box, or when wet with salt water or if injury is likely. In any case, the stats are such that there are no fatalities to discuss from the RLV system.

The thing that rather spoils it often in practice  is the 50m of cascaded 13A extension leads at 230V all the way to the 110V cement mixer,  where the transformer sits sadly in a puddle of dirty water ;-)  That largely defeats the point..

Mike

mapj1 said:

The thing that rather spoils it often in practice  is the 50m of cascaded 13A extension leads at 230V all the way to the 110V cement mixer,  where the transformer sits sadly in a puddle of dirty water ;-)

With a bit of luck, voltage drop might stop the mixer from starting. :-0

voltage drop might stop the mixer from starting.

It is more likely to do so  if you cascade too many 110V leads after the transformer, rather than 230V ones before it - sadly a reason not to do things properly. I have even seen rather nasty adaptors from 110V BS4343 connectors to allow mains to be sent on leads that look like 110V to the casual inspector ;-)

Elsewhere on the planet 400V 3 phase and 230V single phase leads that use connectors actually designed to get wet are used to great effect on building sites for this sort of purpose.

Mike

Hi, thanks for the all replies.  That does help clear things up.  Would most people agree, having some form of  addition protection on the secondary would be good practice depending on application? 

Perhaps, but not necessarily, for example  if it became a quick way to isolate the load without unplugging it to change blades or whatever.
Are you thinking of an RCD ?- and if so what fault path are you expecting to operate your ADS that is not already covered, and who is at risk  of what  sort of accident if it doesn't ?
Mike.

KC said:

Would most people agree, having some form of  addition protection on the secondary would be good practice depending on application? 

RCDs can be used for ADS, but so can OCPDs (see Regulation 411.8.3 of BS 7671).

For the purposes of BS 7671, the OCPD must be selected so that it will effectively disconnect both line conductors (same Regulation number).

Specifically in terms of 'additional protection', Regulation 411.3.3 of , that requires additional protection by RCDs with residual current rating not exceeding 30 mA for circuits supplying socket-outlets, the same Regulation states that it does not apply to reduced low voltage systems, so overall, in general, it's not necessary to provide additional protection on the secondary for such systems.

mapj1 said:

Perhaps, but not necessarily, for example  if it became a quick way to isolate the load without unplugging it to change blades or whatever.

So, yes, exactly, if you can find an RCD that is intended for RLV systems (Rated voltage Un of 110 V) then the question is, what additional risk is to be addressed.

In terms of selecting RCDs, RCBOs or circuit-breakers suitable for use in particular switching operations, see Note (5) to Table 537.4 of BS 7671.

Hi, I was thinking about long cable runs on the secondary, and the thermal breaker on the primary not being able to meet the required disconnect time.

what disconnect time ? - its 55V to earth before its pulled down by the fault current, and probably way less than 50 at the fault, more like 25-30V 

Mike.

what disconnect time ? - its 55V to earth before its pulled down by the fault current, and probably way less than 50 at the fault, more like 25-30V 

Mike.

what disconnect time ?

BS 7671 still asks for a 5s disconnection time for earth faults for RLV systems - 411.8.3 (despite the underlying physics/biology not really requiring it).

I was thinking about long cable runs on the secondary,

If cables are too long you're likely to hit voltage drop problems first (although admittedly those may be overlooked in practice). As a rough rule of thumb, if voltage drop requirements are met (and c.p.c.s are full size) ADS is likely to work for C-type MCBs - B-types should therefore have no problems and fuses should be OK (certainly for 5s disconnection times).

   - Andy.

AJJewsbury said:

BS 7671 still asks for a 5s disconnection time for earth faults for RLV systems - 411.8.3 (despite the underlying physics/biology not really requiring it).

BS 7671 permits 5 s on distribution circuits, and heavy-current circuits, in TN systems (and 1 s for the same classifications of circuits in TT installations). 

The physics/biology doesn't support this either for all possible touch-voltage scenarios, although inside what we used to call the "equipotential zone", in TT systems, touch-voltages should be relatively low, and in TN systems in the "equipotential zone" touch-voltages are likely to be relatively low.

5 s (and 1 s) is really a "pragmatic" time.