42 amp supply to 16 amp socket would you pass it with inspection and testing?

[ericmark]

Most will have guessed the subject with a 10 amp overload they can deliver 41.8 amps when one line is shorted to earth for a very long time without anything tripping, common to find extension plugged into extension so the cable is that long there is insufficient current at 230 volt to open a 10 amp overload, but there is over 40 amps at 55 volt.

Where I work in the main they are used to supply 110 volt lighting, most power tools are battery or 230 volt, it is not a building site, so there is no rules saying 110 (55-0-55 or 64-0-64) volt must be used, but in a building which leaks like a sieve, reduced low voltage lighting is considered better.

Power tools do not present such a problem and the cables are run out for a job, then collected again once finished, but lighting can easy be left on for an extended time, and possibly no one would notice it had failed.

So with inspection and testing of in service electrical equipment would you pass it? And what would be the reason given if failing it, assuming in an as new condition?

 

[SUNRAY]

My view has always been these transformers should have double pole OCP.

One of the places I worked, in the factory prior to transporting a installing on site I assisted with fitting out a roadside pumping station in a glass fibre hut, this sort of thing:

The 240V supply was transformed down to 110V for everything. The company's electrician fitted a 32A RCBO to supply a 6KVA or so 110V transformer followed by a domestic CU with single pole MCB's including 16A MCB for each of the several 16A yellow sockets and of course red and black wiring.

I queried only pole MCB's in the CU and essentially told I was not an electrician and not my business. I spoke to a manager and he didn't understand the problem, so I used a yellow plug with a short length of 1.5mm² flex and linked the unprotected phase wire to the earth and powered up.

With the MCB off the manager, the electrician and myself stood and watched the piece of flex and the wires in the trunking overheating and smoking, .
The electrician was promply instructed to change the damaged wiring and CU for a bigger size to take 2 pole MCB's.

Also they had to put an inspection and modification program in place which was no mean feat as the same method had been used for years in a lot of cabins.

The situation you describe is in my view totally unacceptable. However it is very difficult to classify the problem with a commercially produced device, the likes of which have been in use for many decades, apparently without problems. The demonstration I performed resulted in the problem being fixed...

 

[JohnW2]

My view has always been these transformers should have double pole OCP.

If (as I presume) you're talking about a 'centre-earthed' supply (e.g. 55-0-55), then it is surely not just "your view" but, rather, absolutely essential that any protective devices and isolators (and, ideally, also even functional switches) be 'double pole', isn't it (for the very reason you go on to illustrate with your story) ?

 

[SUNRAY]

If (as I presume) you're talking about a 'centre-earthed' supply (e.g. 55-0-55), then it is surely not just "your view" but, rather, absolutely essential that any protective devices and isolators (and, ideally, also even functional switches) be 'double pole', isn't it (for the very reason you go on to illustrate with your story) ?

Yes indeed it is my view, however the yellow brick manucacturers seem quote happy supplyiong them up to 5KVA without and have done for decades, pressumably without the problems Eric and I have highlighted.

On the larger 'site' versions

they do provide DP MCB's.

 

[bernardgreen]

A centre-earthed supply (e.g. 55-0-55) is in reality a 2 phase supply with a Neutral

A two phase supply requires 2 mechanically linked breakers, one for each phase, in others a two pole MCB

 

[ebee]

Certainly when we were in the EU (if not just in Europe, Harmonisation wise) I was told that any RLV supply could not be mandated on UK building sites or anywhere else. Before that then I guess it could be.

PS yes both poles should have overcurrent protection not just one pole.

 

[Harry Bloomfield]

Certainly when we were in the EU (if not just in Europe, Harmonisation wise) I was told that any RLV supply could not be mandated on UK building sites or anywhere else. Before that then I guess it could be.

PS yes both poles should have overcurrent protection not just one pole.

Some of the smaller transformers, seemed to optionally include a manual reset thermal trip, wired in the mains input.

 

[JohnW2]

A centre-earthed supply (e.g. 55-0-55) is in reality a 2 phase supply with a Neutral ... A two phase supply requires 2 mechanically linked breakers, one for each phase, in others a two pole MCB

Good point. One obviously needs OPD(s) which not only 'break' both 'phases' when they operate but also sense over-current in both 'phases' (regarded as "L & N" by the device).

Do DP OPDs have such functionality or does one, as you suggest, have to mechanically link two SP ones?

Kind Regards, John

 

[SUNRAY]

Good point. One obviously needs OPD(s) which not only 'break' both 'phases' when they operate but also sense over-current in both 'phases' (regarded as "L & N" by the device).

Do DP OPDs have such functionality or does one, as you suggest, have to mechanically link two SP ones?

Kind Regards, John

That is basically what a DP MCB isor are you thinking of a different product?

 

[JohnW2]

That is basically what a DP MCB isor are you thinking of a different product?

I'm not thinking of anything but, rather, 'asking' - do all "DP MCBs" sense both poles as well as switching both ?

 

[SUNRAY]

Some of the smaller transformers, seemed to optionally include a manual reset thermal trip, wired in the mains input.

That's what this whole thread is about.

A (typically) 10A trip on the primary and a short circuit on half of the secondary (55V) allows (230/55)*10A to flow in the secondary and hence 16A connector and subsequent 1.5mm² flex = 40 to 45A

 

[SUNRAY]

I'm not thinking of anything but, rather, 'asking' - do all "DP MCBs" sense both poles as well as switching both ?

Yes a DP or 2P MCB is simply 2 MCB's joined together, they include a tripping link in addition to a handle link.

There are auxilliary contacts available (ususlly ½ module width) to add to the side of a MCB (1, 2, 3 or 4 pole) which I've seen used as a neutral break however they are usually only rated to about 6A and designed to be a control signal to show the MCB has tripped or to restart the timer in a λΔ starter for example.

However it seems DP RCBO's in a single module width may not trip on neutral over current but rely on the RCD action and don't switch the neutral.

 

[ericmark]

As said larger step down transformers have twin or three pole overloads on the output, be it split or three phase supplies. I have seem problem like @SUNRAY describes where a centre tapped earth transformer was supplied to replace a 110 volt to earth transformer used with a control circuit in a machine, which as @SUNRAY describes resulted in one side not being overload protected to earth.

However I am not talking about this, I am talking about yellow bricks, and as again @SUNRAY shows they can deliver 40 amp to earth from a 16 amp socket.

To use a classification often refereed to, are these transformers dangerous, potentially dangerous, or should there be simply a warning on them, remember fire is dangerous, not so much looking at shock risk as to fire risk.

To have a transformer with two 16 amp sockets on one 20 amp double pole MCB may be technically exceeding the rating of the socket, but in the main 1.5 mm² cable can take 20 amp without melting, however it can't take 40 amp without melting. To fit two 20 amp overloads on the output even if not linked would remove the fire risk, but a 10 amp on input only protects the transformer primary winding.

So when doing the inspection and testing of in service electrical equipment, should transformers without protection on the secondary fail?

 

[Ragnar_AT]

1P+N MCBs are a thing but extremely uncommon in the UK. These switch the neutral but do not monitor for overcurrent. They‘re often used in countries that traditionally had TT supplies with poorly earthed neutrals.

 

[SUNRAY]

As said larger step down transformers have twin or three pole overloads on the output, be it split or three phase supplies. I have seem problem like @SUNRAY describes where a centre tapped earth transformer was supplied to replace a 110 volt to earth transformer used with a control circuit in a machine, which as @SUNRAY describes resulted in one side not being overload protected to earth.

However I am not talking about this, I am talking about yellow bricks, and as again @SUNRAY shows they can deliver 40 amp to earth from a 16 amp socket.

To use a classification often refereed to, are these transformers dangerous, potentially dangerous, or should there be simply a warning on them, remember fire is dangerous, not so much looking at shock risk as to fire risk.

To have a transformer with two 16 amp sockets on one 20 amp double pole MCB may be technically exceeding the rating of the socket, but in the main 1.5 mm² cable can take 20 amp without melting, however it can't take 40 amp without melting. To fit two 20 amp View attachment 343328 overloads on the output even if not linked would remove the fire risk, but a 10 amp on input only protects the transformer primary winding.

So when doing the inspection and testing of in service electrical equipment, should transformers without protection on the secondary fail?

There is no easy answer and there are other questions involved, such as what lighting load do you have on yellow bricks as technically they are designed for intermittant tool use and the continuous rating is usually around 50%, from memory I think my 1.5KVA is 800VA and 3.3KVA is 1.8KVA and I know from experience a 1.5KVA will trip it's thermal overload when running at 1KW continuous. IE: Lighting festoon hired from a plant hire company with 40*25W for an outdoor carol concert, luckily I'd thought to take the 3.3KVA as a backup and even that was running hot outdoors in December.