IEC 240Vac Inlet, 110Vac Center Tapped Supply ?

[Nitra]

hi

I am having to justify (for safety) - if a 110Vac center tapped supply is plugged into a 240Vac IEC C13 Inlet on an instrument (with internal tapping changed to 110Vac).

In most instances the inlet will have a single fuse and a single pole switch.

The L.V. Directive I believe requires that both lines are switched and fused.

What words do I need for justification ?

TY

[helius]

It's more common for devices with switchable taps to use a parallel/serial configuration, not just a center tap. The reason is that when operating in 120V mode, it requires twice the primary current as in 240V mode. When the taps are switched to 120V mode and the device gets plugged in to 240V, the transformer is generally not the part that fails: the secondary side simply supplies twice the design voltage and causes failure of downstream components (like overheating of linear regulators, which may short out, and then cause cascading failures along the regulated power rail).

[Nitra]

Missed the point - or a bad explanation by me.

The 110Vac center tapped supply is from the external source e.g. 240/110Vac Tool Transformer, or wall outlet (typically yellow three way outlets).

Test equipment is set for 110Vac - so thats ok, but the internal fusing and isolation switch should be on both lines.

[IanB]

This is confusing, since the internal switch and fuse arrangement of the test instrument is fixed and is not something you can change. It is presumably safe and meets requirements for the domestic 240 V mains supply.

Now instead you choose to power the instrument from a 55-0-55 V tool transformer where I think the center tap is earthed? The danger here is that a short circuit to the grounded chassis inside the instrument could lead to an over current situation on either leg of the supply. However, the same danger could exist with a 240 V supply if the L and N lines were inadvertently reversed (wrongly wired cable)? Either way, the over current problem could be protected against by fuses in the supply, rather than the fuse inside the instrument.

Specifically, what is the potential danger that is at issue here?

[Ian.M]

If the equipment is safe (when set to 220-240V input) for use in Europe, where in many countries the L and N conductors are effectively interchangeable due to non-polarised wall sockets that can accept a Schuko plug in two orientations, its safe to run from a center grounded 110V supply (when properly configured for that voltage),  as long as the supply over-current protection will shut off both conductors and does not have a rated current exceeding the 16A max capability of the Schuko system and IEC C13 inlet.

[Gregg]

If the 110V windings of the transformer are completely isolated from the 240V winding and neither pole of the 110V is grounded; you can ground one leg of the 110V and call it neutral for your purposes and call the other leg the line which would be connected to the single fuse.
On the other hand if it is an auto transformer with one leg of the 110V connected to one leg of the primary, you have to be careful as to which leg of the 110V you use as neutral which should be the same as the neutral of the 240V side.  Your description of the transformer being center tapped could apply to either a true isolation transformer or to an auto transformer.

[IanB]

It is a center-tapped isolation transformer used for safety on construction sites for power tools. The center tap is grounded so that each hot leg is at 55 V AC, thereby minimizing the shock risk.

[helius]

Sorry, I see that I misread your question.
I think the idea of the 55-0-55 worksite transformer is that even if one of the phases becomes live on the tool housing, it remains touch safe. So there is no direct danger of electrocution from the equipment being fused on only one side. It may not be so kind to active probes connected to the oscilloscope.

[Nitra]

There may not be any danger. But...........

I am still obliged to comply with the UK L.V. directive.

The difficulty is that I often work in areas where 240Vac working voltage is forbidden but the proprietary equipment I use is designed for 110Vac/240Vac Line, Neutral and Earth.

With 110Vac center tapped earth, to have different levels of protection between Line 1 and Line 2 e.g.. an overcurrent on L1 will trip instrument protection, while an overcurrent on L2 will blow the supply protection seems awkward at best to explain.

The 110Vac supplies are standardized at 16Amp somewhat larger than the amp or two of an instrument.

Anyhow thanks everyone for the advice.

[Zero999]

Yes fuse both lines. If it's a pre-packaged transformer, it should have all the necessary protection built-in.

55VAC is a high enough voltage to give a shock. It's just unlikely to be lethal, in dry conditions. However, if it's wet, there's a significant risk of a fatal shock.

[IanB]

If the job site transformer is designed for safety reasons, might it also have an RCD built in?

If the transformer is designed to lower the risk of electrocution, then the risk is lowered no matter what you choose to power from it.

[Zero999]

If the job site transformer is designed for safety reasons, might it also have an RCD built in?

If the transformer is designed to lower the risk of electrocution, then the risk is lowered no matter what you choose to power from it.

I haven't seen one with RCD protection built-in. The 110V centre tapped to earth standard, came into being, before RCDs were common place. Personally, I'd prefer to use 230V, with an RCD, than 55-0-55V with no RCD.

[Ian.M]

However if you *MUST* use 55-0-55V site power, and need to feed 110V to multiple instruments, consider using a 120V rewireable inline GFCI imported from the US (they call RCDs GFCIs), feeding an imported US socket strip (with breaker for overload protection), and US 3 pin (NEMA 5-15) plug to C14 trailing socket leads for your individual instruments.   It avoids putting 110V on BS1363 or European socket strips,  and if you attach the C14 end of the lead to the equipment with a lanyard to a large self-adhesive cable tie base, vastly reduces the risk of equipment configured for 110-120V being plugged into a 240V wall outlet.

[Zero999]

However if you *MUST* use 55-0-55V site power, and need to feed 110V to multiple instruments, consider using a 120V rewireable inline GFCI imported from the US (they call RCDs GFCIs), feeding an imported US socket strip (with breaker for overload protection), and US 3 pin (NEMA 5-15) plug to C14 trailing socket leads for your individual instruments.   It avoids putting 110V on BS1363 or European socket strips,  and if you attach the C14 end of the lead to the equipment with a lanyard to a large self-adhesive cable tie base, vastly reduces the risk of equipment configured for 110-120V being plugged into a 240V wall outlet.

I agree with using a GFCI/RCD.

US sockets aren't normally used in the UK, even for 110V equipment. They aren't robust enough for use in the hazardous areas, where a 110V supply is mandated. Even the 230V British plug is not recommended for use in industrial environments.

The correct standard is IEC60309.
https://en.wikipedia.org/wiki/IEC_60309
https://www.mkelectric.com/en-gb/Products/WD/IngressProtected/commando2013/Documents/MKCommando2013RangeUpdateBrochure.pdf

The 110V connectors are yellow.

[Ian.M]

Yes, but within a test equipment rack you'll have fun fitting in a IEC60309 110V distribution board!   I suggested the US connectors because they are intended and rated for 120V  operation, vastly decreasing the risk of a F***idiot plugging in equipment jumpered for 110-120V to 240V.   I suppose you could use a rack mount IEC C14 socket strip for its shrouded connectors, but that increases the risk of said F***idiot unplugging one of the C13 to C14 leads and plugging it into 240V via their kettle lead!

[Nitra]

Thanks to everyone.

I am in process of ensuring all 110Vac (Yellow) to IEC C13 lead sets have an inline RCD fitted -

[Ian.M]

That's the PowerBreaker J62-T for 230V operation (orange case).  Will it operate within the rated time with its control circuit powered from 110V?  You probably should be using the 110V PowerBreaker J62-Y (yellow case).

[Nitra]

Yep, the picture is generic only (RS components), I have the correct one for 55-0-55 Vac