New Fuse Box Needed for 110v Electrics

[Marc Harris]

Hello Guys,

I have a strange, yet accurate title: I am the Vintage Member for the WBCCI. That is the club for people who own Airstreams. (the aluminium bodied, curved caravan, or as we call them: A Travel Trailer) In my case, the Euro branch. This means that members who have problems come and ask me. There is a problem I've considered for a while.

Many importers of vintage Airstreams fit transformers in order to use the 110v system in the trailer. This is most often because the trailer may contain 110v appliances. A fridge or often an AirCon system. This means firing up the vintage wiring which is not generally a problem in itself. US wiring is very robust and bomb proof and will last for decades.

The problem I want to solve is fuses and particularly RCDs: The 110v side of transformer is not protected by the RCD which are fitted into the 240v input (this is typically via a small fusebox, the type of thing you would fit in a garage extension) The fuses which are fitted in the Airstreams are only MCBs and provide no protection to the system. How can I solve this?

Can we use UK fuses, such as a UK RCD inline on the 110v feed

 

[westward10]

You require double pole protection and UK devices will operate on 110v.

 

[Deleted member 26818]

If the 110V supply is 55V - 55V additional protection by means of an RCD is not required.

 

[davesparks]

Are you using yellow site transforms or are they something specific to the job?

Is the output side of the transformer referenced to earth?

 

[GeorgeCooke]

Why are you fitting 110v transformers when the voltage used in the US is 120v?

 

[GeorgeCooke]

Why not import US RCDs, known as GFIs over there? If you are using US NEMA sockets you can get them with GFIs built in, they are a requirement in bathrooms.

 

[Marc Harris]

Gentlemen,
I’m gratified by your assistance. I’ll go through your responses 1 at a time. Firstly, I use a toroidal transformer supplied by Airlink in Essex. Very reliable bomb-proof kit. I know of some people who have used the yellow site units. I warn them off strongly as they aren’t intended for continuous use. The 110v appliances work very well despite the reduce in Hertz rate from 60(US)to 50Hz in the UK.
I’m sure the trans is referenced to earth but not sure if it’s 55+55.
I have found a earth leakage circuit breaker on Amazon.com. Seems perfect as it protects to 30amp in a 3kva circuit but it’s single pole. Why does it need to be double pole?

 

[davesparks]

The 110v appliances work very well despite the reduce in Hertz rate from 60(US)to 50Hz in the UK.
I’m sure the trans is referenced to earth but not sure if it’s 55+55.
I have found a earth leakage circuit breaker on Amazon.com. Seems perfect as it protects to 30amp in a 3kva circuit but it’s single pole. Why does it need to be double pole?

You need to find out if and how the secondary side of the transformer is earthed and the type of rcd required before sourcing the RCD.

If the output of the transformer is not referenced to earth then there is unlikely to be much point fitting an rcd at all.

Double pole fusing, mcbs and rcds will be needed if neither pole of the supply is referenced to earth or if it is centre tapped to earth as both conductors are line conductors.
Single pole devices are only suitable if there is only one line conductor.

 

[Bobster]

You need to find out if and how the secondary side of the transformer is earthed and the type of rcd required before sourcing the RCD.

If the output of the transformer is not referenced to earth then there is unlikely to be much point fitting an rcd at all.

Double pole fusing, mcbs and rcds will be needed if neither pole of the supply is referenced to earth or if it is centre tapped to earth as both conductors are line conductors.
Single pole devices are only suitable if there is only one line conductor.

You've contradicted yourself in that post Dave.

Even if the secondary has no reference to earth, an RCD will still function if there is a current imbalance. Which there would be if someone grabbed a live conductor through capacitive coupling. Not sure if it's enough to divert 30mA but if it was, a functioning RCD would operate.

Seen a few setups in labs that use an isolated transformer, then have 5mA RCDs on the circuits.

 

[GeorgeCooke]

I repeat, the US devices are 120v, not 110v.
Are your transformers auto transformers or fully isolated? This is important.

 

[davesparks]

You've contradicted yourself in that post Dave.

Even if the secondary has no reference to earth, an RCD will still function if there is a current imbalance. Which there would be if someone grabbed a live conductor through capacitive coupling. Not sure if it's enough to divert 30mA but if it was, a functioning RCD would operate.

Seen a few setups in labs that use an isolated transformer, then have 5mA RCDs on the circuits.

Have I? You'll have to give me a clue as I can't see it for ----?

Is that capacitive coupling between the windings of the transformer? Assuming the supply and outgoing wiring are physically seperate I can't see where else it could be coupled.

 

[Bobster]

Have I? You'll have to give me a clue as I can't see it for ****?

Is that capacitive coupling between the windings of the transformer? Assuming the supply and outgoing wiring are physically seperate I can't see where else it could be coupled.

If the output of the transformer is not referenced to earth then there is unlikely to be much point fitting an rcd at all.

Double pole fusing, mcbs and rcds will be needed if neither pole of the supply is referenced to earth or if it is centre tapped

Reading again, I imagine its a typo and you meant either rather than neither.

Capacitive coupling through the human body.

 

[Bobster]

I repeat, the US devices are 120v, not 110v.
Are your transformers auto transformers or fully isolated? This is important.

It's within 10%.

I've seen plenty of places in the US that have 110v

 

[GeorgeCooke]

It's within 10%.

I've seen plenty of places in the US that have 110v

When? 110v was phased out over 50 years ago. The current standard is 120v plus or minus 5%. i.e. 114 to 126v.

 

[davesparks]

When? 110v was phased out over 50 years ago. The current standard is 120v plus or minus 5%. i.e. 114 to 126v.

It's probably like the UK changing from 250V to 240V and now 230V, the actual transformers are still outputting exactly the same 250V they always were, it's just the paperwork that changed.

 

[davesparks]

If the output of the transformer is not referenced to earth then there is unlikely to be much point fitting an rcd at all.

Double pole fusing, mcbs and rcds will be needed if neither pole of the supply is referenced to earth or if it is centre tapped

Reading again, I imagine its a typo and you meant either rather than neither.

Capacitive coupling through the human body.

I see it now, I was trying to say that double pole protection is needed for both conductors because they are both line conductors when the transformer output isn't referenced to earth, or if its centre tap earthed.

 

[Deleted member 26818]

From what I understand in America.
Voltages and types of supply vary from STate to State, town to town and even building to building.
My information, is that there are still buildings in New York that have dc supplies.
To suggest there is a national standard, is a bit like saying everyone agrees on Brexit.

 

[Lucien Nunes]

The neutral at the output of the transformer should be referenced to earth (not the centre-tap), as that is what normal US appliances and single-pole-switched lighting circuits are designed for. Thus, only single-pole MCBs would be required. UK-type RCDs and RCBOs will work correctly in the event of leakage, although the test button may not work because of the lower voltage. UK-type MCBs will work correctly in the event of overcurrent or short-circuit, although the short-circuit current at the output of the transformer may be limited, making larger breakers relatively slower to trip in the event of a fault.

Yellow site transformers are continuously rated, but for a lower output than their 'tool rating' which is the one by which they are sold. The continuous rating is usually given on the data plate. However, because it is desirable in the trailer to have the neutral earthed, not the centre-tap, and a yellow tool transformer should never be modified in this way, I would agree they are a bad choice.

Because of the transformer regulation, which is likely to be around 5% for a 1-2kVA unit, a 110V rated secondary will output around 115V when lightly loaded as it will be most of the time, e.g. at night when the aircon is off and only electronic goods are switched on. Whereas, a 120V transformer will output 126V. So, for a 120V circuit, I'd probably opt for a 110V-rated transformer.

In any case, I think we talk about 110V and 120V as a general 'class' of things without meaning an exact voltage. I would often say 'a 110V motor' or 'a 120V motor' indiscriminately, to distinguish it from a 12V one or a 230V one.

The US uses a wider variety of supply configurations than the UK, but they are all recognised by the national standards. Some localities may have one or other three-phase system more readily available, according to the age of its infrastructure, for example. The UK site supply arrangement, 55V-0-55V with centre-tap earthed, is not used for USA mains power. I understood that the last public DC supply in NYC was switched off by Con Ed in 2007.

 

[Deleted member 26818]

Yes Con Ed did disconnect their last dc customer in 2007.
However many of the older buildings have had rectifiers installed to convert to dc (primarily for lift winding motors).
Also many buildings, such as the Dakota Apartments had their own electricity supplies.
Not sure if D.C. still has dc (it did in 2012), though it has the same problem with having dc lift winding motors.
From what I understand, San Francisco still has dc in the area covered by the first public electricity supply company.
Again this is primarily for dc lift winding motors.

 

[Marc Harris]

Gentlemen,
there were too many and complex to ask but since your questions all related to the transformer I thought my next stop should be tech support at Airlink. Perhaps the answer will be self-explanatory. I have been advised that a 240v 32Amp Dual Pole RCBO is the kit I need to protect the 3Kva 110v system.
Thank you so much for all your answers. I think I need to expand my knowledge beyond V over I R