Isolation transformers, use of.

[Nick]

Just looked at the IEE 60755 regs for RCD trip times, and:

• Minimum break time: immediate
  
  
• Maximum break time: 200 ms for 1× I_Δn, 150 ms for 2× I_Δn, and 40 ms for 5× I_Δn
  

i.e. with a HUGE fault, there could still be up to 40mS before tripping, and for a minor fault (at rated trip value), up to 200mS.

Either of these can be "exciting", and should be avoided

[AlanBeckett]

Interesting and fascinating.
Back in the '70s, at Plessey Beeston, the Lab Benches were fed from individual Isolating Transformers. The Benches were fitted with 5A and 15A Sockets to stop people pinching anything useful. There were a number of Emergency 'Stop' Buttons on each Bench. I don't know where the Earth Pins were connected to or whether the Supply was really 120-0-120.
We just plugged everything in - Gear we were developing and Test Gear - as normal.
Never had any problems.

Alan

[raylec]

An isolation transformer galvanically isolates the earth connection from the supply. It should only supply the equipment being worked upon. Nothing elase should be connected.only one piece of equpment. If the set is an AC/DC model it will most likelly have a live chassis,it will be connected to both sides of the input supply,the result that if you come into contact with any of the circuitry, you will not receive a shock, or if you accidently put an eath to the chassis,will not cause a dead short.
You can connect either pole,Ie there is no L or N.
If you dont understand then you should learn more about basic theory.
An RCD feeding an isolating transformer is pointless,and will not provide any protection.
The poles from an isolation transformer will not be 120V to earth as has been suggested, but may be any voltage from 0-240v depending on leakage(capacitive) this residual voltage will not(or should not) supply any dangerous current to earth. The supply is known as a floating supply. Floating supplies are not allowed to feed any appliance or for any general supply (IEE regs) All supplies of electricity must be earthed. Thats why an isolating transformer only feeds one piece of apparatus only, which must not be connected to any earth.
Portable tools for outdoor use are feed from a 110v transformer, some people have called this an isolating transformer,however this is a double wound with a centre tap to earth, so that a shock from either side will be 55v
This is all basic electrical theory,if you dont understand leave well alone.!

[Nick]

An RCD feeding an isolating transformer is pointless,and will not provide any protection.

All good stuff and totally agree - which is the concern some of here are expressing. I slightly query your dismissal of the use of an RCD - indeed it provides no secondary protection, however it still provides primary circuit fault protection. 

[Skywave]

What are doing is creating your own mains supply, completely floating wrt what comes into your house. You are using a double-wound TX to do so.
Exactly: that's the whole idea. And a balanced 120-v. - 0v. - 120-v. supply too, fed from an RCD with suitable fuses fitted also.

You are also separating your local bench "earth" from mains earth and any extraneous earth such as water pipes. Hence under fault conditions . . . .
"fault conditions": such as ?

. . . . dangerous potentials can exist between your "earth" and any other earthed metalwork. If you can guarantee your workshop to be an earth free zone this is a recognised way of working under certain specialised conditions. Otherwise it's dangerous.
As you have said earlier, there are many ways to hurt yourself by misuse of electricity - especially A. C. mains potential. It's all a question of balance and assessed risk. People such as you and I - who have the necessary experience and knowledge - are quite capable of making such a risk assessment - and acting accordingly.

Judging by previous threads in various forums there is widespread misunderstanding about how to create a safe working environment. Much of this misunderstanding concerns isolating TXs but (if you'll pardon the pun) it cannot really be considered in isolation from other aspects of safe working.
O.K., point taken. Although I raised this thread with its chosen name, I was (and am) aware of the answer that that question asked. Call me a devil's advocate if you like, but electrical safety is important and it affects us all.  I just wanted to produce a meaningful conversation amongst members on this topic of 'isolation transformers' (for the very reasons you have mentioned above) and then to lead on to my specialised method of using one.

Al.

[Mark Hennessy]

I can give an example of a seemingly counter-intuitive use of isolation transformers:

A type of OB vehicle that I am familiar with uses them. There are 4 in total, as the truck can be powered from a pair of 13A BS1363 outlets, or a single 63A C-form BS4343 outlet. Each of those has a transformer, so that's 3 so far. The 4th transformer is another 13A inlet called "night supply" or similar, and this is used to keep certain essential things alive while the truck is parked up overnight. Of these transformers, 3 of them are rated at 12A, and the other is the full 63A. They are massive, heavy and scarily expensive...

Now, inside the vehicle, one side of the secondary of each of these transformers is connected to the chassis of the vehicle, and in turn to mains earth!

So the outputs from these transformers are not isolated at all...

Why on earth would they go to the expense of fitting these - not to mention the cost and weight penalty - when they are seemingly doing nothing at all?

Well, nothing is simple...

First of all, there are RCDs to protect the operators from receiving electric shocks from faulty equipment. If the secondaries of the isolation transformers were left floating, the RCDs wouldn't work.

OK, so I understand the need for RCDs, and I understand that we need to create our own "live" and "neutral" for those to work. But still, why bother with these transformers???

To answer that, one needs to consider the leakage current of all the gear on a truck. There are countless switched-mode power supplies, and the input filter components of switched-mode power supplies cause leakage to earth. Not a problem in isolation (bad pun!), but it all adds up, and it might well be enough to trip an RCD in the venue - especially if the venue power supply already has its share of leaky devices. This, BTW, is a big problem for places like call-centres and server rooms, where RCDs just aren't practical unless you partition off the supplies so the RCD only supplies a small number of devices...

So, whatever leakage current all the gear on the truck has, the isolation transformer removes that from the point of view of the venue's supply. And the local RCDs on the truck are doing their job, and are under control of the operators - no need to contact the venue's electrician to restore power. Neat...

Of course, the truck operators have to consider all the different types of mains supply that they might encounter at a venue (TN-S, TT, TN-C-S/PME, etc), and whether they need to deploy an earth spike. It's complicated. I have a 50 slide Powerpoint presentation here that goes over all this - it's a day of training just to introduce the concepts.

As an aside, it's worth noting that cascading RCDs is obviously troublesome because you can't predict which one will fire first. My bench is plugged into a standard ring-main socket in the cellar, so in theory, a problem on my bench might trip the house RCD before the bench RCD. I'm aware of that, and I'm aware that I should run in a cable back to the consumer unit to pick off a non-RCD protected feed so that I can have a small consumer unit in the cellar to provide a dedicated non-RCD feed to the bench - but these days that involves paying for sign-off, etc. Presently, it's not a safety issue; should the house RCD trip first, it's just mildly inconvenient. The lighting isn't protected by the house RCD (installation is approx 16 years old, I believe), so the place isn't plunged into darkness. I do intend to fit some emergency lighting to deal with complete power cuts, but they are so very rare here that I keep forgetting to get around to it...

Anyway, hope some of that is interesting...

Mark

[Skywave]

An isolating transformer only feeds one piece of apparatus which must not be connected to any earth.

From an earlier post of mine:

That transformer is internally configured for 120v. - 0v. - 120v. bi-phase output with the 'E' / 0v. centre point connected to supply earth inside the commercially-built isolation transformer unit which has full Government electrical safety  approval.

Note: That 'earth through' arrangement has now been disconnected my me.

Al.

[Mark Hennessy]

What are doing is creating your own mains supply, completely floating wrt what comes into your house. You are using a double-wound TX to do so.
Exactly: that's the whole idea. And a balanced 120-v. - 0v. - 120-v. supply too, fed from an RCD with suitable fuses fitted also.

But if, as stated in post #5, you have not connected the centre-tap to mains earth, then you have not created a balanced supply - you have created a floating supply with the ability to make the metal cases of any class 1 equipment plugged into it float up to 120V, as explained in post #10.

Moreover, if a "phase" of this supply is connected by accident to mains earth, then I don't immediately see how an RCD between the transformer secondary and the sockets can detect an imbalance. Well, not until you touch the metal case of a class 1 device (which will be at 120V at this time).

The earth of the outlets must be firmly connected to mains earth for this arrangement to be safe. Simple as.

Out of interest, what does this transformer supply? Just DUT, or DUT plus test equipment?

[Skywave]

Interesting and fascinating.
Alan.

That is the essential idea of this thread: my attempt to stimulate a meaningful and an objective discussion on an important topic. As others here have pointed out, the use of isolating transformers is often not fully understood. So this thread has two objectives: one, to stimulate a discussion on the use of such transformers and secondly, an illustration of an alternative way of using one.

Al.

[Mark Hennessy]

An isolating transformer only feeds one piece of apparatus which must not be connected to any earth.

From an earlier post of mine:
That transformer is internally configured for 120v. - 0v. - 120v. bi-phase output with the 'E' / 0v. centre point connected to supply earth inside the commercially-built isolation transformer unit which has full Government electrical safety  approval.

Note: That 'earth through' arrangement has now been disconnected my me.

Al.

Which had full Government electrical safety approval until you modified it.

What "raylec" is referring to is the same as me - the use of an isolation transformer to allow safe connection of test equipment to a device under test.

Which is quite different to your use - which is creating a centre-taped supply that ensures that the supply could only be 120V w.r.t. mains earth (well, that was the case before you modified it).

Whether it's worth the effort of making a balanced supply is debatable, but if you have the transformer, and don't mind the space and magnetising current it consumes, then why not? You still need a separate isolation transformer for the use raylec and I refer to, so if you have one, and are aware of that, then no problem