110volt transformers with centre tap 55volts???? confused?

[ericmark]

I will agree many RCD's don't have a problem as to voltage. But that does not mean all RCD's will work. I would guess the auto resetting type would have a problem?

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• ericmark
• 6 Dec 2008
• 34

We should test all RCD's on fitting but that is also a problem with many meters not able to work with 55-0-55 supplies. I have not tested any but one should really double up on tests and try both legs to earth not just one. With the testing such a problem I would want to use a RCD designed for the voltage being used.

The 55-0-55 supply does cause dangers I have seen more than once where a transformer had the centre tap connected to earth but the machine it was fitted into was designed to have line and neutral not split phase. As a result no fuse in one of the supplies and also no switches. This is a real problem with equipment imported from USA.

It is far too easy to say "Yes you can do that" and then find there is an exception and so the semi-skilled guy has done something completely unexpected. As far as I remember with farms one does have to be more careful as cows can be killed with just 25 volts. Where we consider 50 volts as being limit for humans. Hence why I am being very guarded with replies.

 

[Paul_C]

6 in considering these Qs. what is USA site practice or even domestic USA practice with lawnmowers etc

As mentioned already, the 120V used in the U.S. has one side of the supply grounded, so it's a different arrangement than the 110V CTE used in the U.K.

With a single-phase supply, the 120V will normally be derived from one side of 3-wire 120/240V system. With 3-phase, it's between one phase and neutral of a 120/208V wye system (or sometimes from one of the two 120V legs of a 240V 4-wire delta supply).

The GFCI has been used increasingly since the early 1970's, it being required by the American NEC (National Electrical Code) for more and more locations over the years, starting with pool areas, then bathrooms, all outside outlets, kitchens, garage/work areas etc. The GFCI is specified to trip at a considerably lower leakage current than the current 30mA RCD standard here, somewhat less than 10mA in fact.

GFCI's can be incorporated with an excess-current circuit breaker at the distribution panel (equivalent to the U.K. RCBO), or GFCI receptacles are available, generally with feed-through terminals so that regular outlets "downstream" can also be protected.

GFCI plugs similar to the RCD plugs found here can also be found fitted to lawnmowers and similar appliances.

 

[franklymdear]

Hi I was following the trail about centre tapped transformers. In the comments about all switches etc being double pole, which I understand, does this apply to an emergency stop, which is there to bring a machine to halt in the event of a dangerous situation arising?

Thanks

 

[stillp]

Hi I was following the trail about centre tapped transformers. In the comments about all switches etc being double pole, which I understand, does this apply to an emergency stop, which is there to bring a machine to halt in the event of a dangerous situation arising?

Thanks

What has that to do with centre-tapped transformers? Emergency stop circuits generally interrupt all live conductors though.

 

[Mursal]

Most are normally closed (NC) and when you hit the button you break (open) the circuit.

Just one example:
http://www.ebay.co.uk/itm/40MM-PANE...ls_Supplies_Electrical_ET&hash=item3f1b45b650

 

[franklymdear]

Thank Mursal,

But, as we have live 55 V AC on both sides of the control circuit, do we need a 2 pole NC emergency stop. Opening one side of the control circuit should stop the machine, but would leave the other side of the control circuit still at 55 V to earth.

Regards

 

[ricicle]

Emergency stop circuits on hazardous equipment are usually wired to safety relays such as Pilz units. Wiring them without these units (just to contactors) is generally unacceptable under the machinery directive as there is no monitoring.
These safety relays interupt the control circuit(s) going to the power contactors, actuators etc.
The 110v in a general control panel should be earthed on one pole which is not the same thing as a centre tapped 110v.

 

[franklymdear]

Hi Stillp,

It is related to the subject of centre tapped transformers, as these are widely used in the design of machine control systems (motor controls etc). In a normal 110 VAC circuit with one leg to earth (N) and one 110 VAC (L), the emergency stop is wired to break the live leg, stopping the machine.

However, where a centre tap control transformer is used, the e-stop would only break the one side (+55 VAC), leaving the other side still at -55VAC. If the circuit had any sneaky earth connection, then in theory, the E-stop might not stop the machine.

As per your comment, the E-stop would need to breal all live conductors, and hence would need to be 2 pole, NC.

Thanks

 

[ricicle]

Why would you want to use a centre tapped 110V for a control circuit.

See my post above !

 

[stillp]

Hi Stillp,

It is related to the subject of centre tapped transformers, as these are widely used in the design of machine control systems (motor controls etc).

Not in my experience. It's a bad design principle due to the possibility of earth faults causing unwanted energisation of relays or contactors.

 

[franklymdear]

Thanks to all for the replies. I can see that the centre tapped 110 VAC is NOT a good design for control, except that it takes the risk of touch potential down to 55 VAC.

However, the issues with control circuits as you have both stated above confirmed my inital suspicions.

I have seen many centre tap 110 VAC control circuits, and was looking at the safety related issues, so your comments have all been really helpful.

Thanks

 

[ericmark]

The centre tapped transformer does present some problems as to Estops as one does need a duel pole safety relay.

As to duel circuits this is required where failure to stop would cause irreversible damage.

Many of the cheaper systems have normally open contacts which are closed to run with a timer between both circuits so if they don't work with a few microseconds of each other the relay will drop out.

Some have one normally open and one normally closed circuit so something cutting the cables could never cause the relay to hold in.

I think from memory most the safety relays are duel pole as common for one set of contacts to open the control circuit and the other to operate a main contactor dropping out all power.

The only not duplicated item is the safety relay it's self.

It really is a design nightmare where cutting power to some items can cause more danger than not cutting power and I know on the machines I was building we had two air supplies one on the Estop the other not on Estop as cutting the air could dump one ton of concrete on the floor.

I have tried to consider where the centre tap would cause a problem and I suppose 55 volt may be enough to hold some relay in but most 110 volt control circuits tend to work on the IT method with neither line being fixed to earth. Often there is also an earth leakage monitor to show if either leg should become grounded.

After the burn out of a machine due to a centre tapped transformer being supplied in error I am now very careful to ensure control transformers are not centre tapped.

One of the big problems is stuff from the USA. We had a fleet of lighting towers which also had 110 volt sockets and the generator was centre tapped. 110 - 0 - 110 with two sockets off one and two from the other with lights having 240 volt supply.

Had to blank off all the 110 volt sockets. Be it from China or USA or Germany there are many items which don't comply with the rules. The German one caught us out seems they were export models so most the safety items were missing. As to if selling to EU is considered as export is another question.