Connecting the Block AIM 5.0/2.0 auto-transformer to UK three-wire plug

I am not electrically very knowledgeable person. At the moment I am trying to connect a Block AIM 5.0/2.5 Auto-transformer to a piece of very sensitive medical equipment. The auto-transformer was purchased to regulate and supply a constant 220 V (to avoid any fluctuations in the voltage as the medical equipment is very sensitive and recommended to run only at 220 V) to the equipment here in the UK.

Now, very recently the company that checks our equipment for electrical safety has labelled this auto-transformer as FAILED due to "No Earth".

Please see the picture where there are only two input (L and N) wires and two output (L and N) wires connected. The ground (earth) wire of both input and output are left open. Now, what and how can I make this device electrically earthed (or have it pass the electrical safety test)?

Thanks a lot.

Product specification: Block AIM 5.0/2.0 Autotransformer.

simulate this circuit – Schematic created using CircuitLab

Figure 1. An isolating transformer and a non-isolating auto-transformer.

At the moment I am trying to connect a Block AIM 5.0/2.5 Auto-transformer to a piece of very sensitive medical equipment.

That's going to have high safety specifications.

The auto-transformer was purchased to regulate and supply a constant 220 V (to avoid any fluctuations in the voltage as the medical equipment is very sensitive and recommended to run only at 220 V) to the equipment here in the UK.

The transformer will not "avoid fluctuations". Its output voltage will fluctuate exactly in proportion to the input voltage.

It is much more likely that the transformer has been recommended to reduce the UK 240 V supply to 220 V to suit the equipment. This is done by setting the 'tap' or tap-off point on the auto-transformer 220/240 = 11/12 of the way up from the bottom of the coil as oriented in Figure 1. That means that it is effectively on a European supply voltage and will tolerate normal fluctuations in supply.

Now, very recently the company that checks our equipment for electrical safety has labelled this auto-transformer as FAILED due to "No Earth".

This is, presumably, because the equipment to be powered requires an earth.

Please see the picture where there are only two input (L and N) wires and two output (L and N) wires connected. The ground (earth) wire of both input and output are left open. Now, what and how can I make this device electrically earthed (or have it pass the electrical safety test)?

The transformer doesn't require an earth because it is fully isolated. It's the load that requires an earth.

simulate this circuit

Figure 2. Standard industrial practice.

If this were an industrial application the solution of Figure 2 would be acceptable. Because it's a medical application you shouldn't be prepared to take that risk and run on uncertified equipment.

Your equipment test company should be able to advise or the medical equipment supplier.

• Thank you very much for the answer. You are absolutely correct. It indeed is used to reduce the 240V to 220V. Thanks again for your kind help :) – Labby Apr 8 at 9:26
• You're welcome. Don't forget to accept the best answer if there is one that solves your problem. – Transistor Apr 8 at 10:47

Essentially, you just connect the incoming earth wire to the outgoing one, rather than just chopping them off and leaving them floating around as at present.

If it were just domestic equipment, I might suggest ways to do it. But it's medical equipment, and doing it safely and correctly is important. Hand it to a qualified electrician and get them to do it properly.

• Thank you very much. I believe it is best for us to check with the manufacturer :) – Labby Apr 8 at 9:29

Autotransformers do not need an Earth Bonded ground connection, but your appliance may need it.

simulate this circuit – Schematic created using CircuitLab

Orange or Yellow should be the proper gauge. Strip 1cm of insulation and twist until firm. and wrap electrical tape around to prevent turning. The other leads need to be shortened to allow the gnd leads to get close to each other.

• Thank you a lot for the suggestions. I really appreciate your help :) – Labby Apr 8 at 9:28