11-02-2015, 11:58 AM
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
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
