Isolatings mains power in laboratory enviroment

Recommended practices

ains power isolation in laboratories is mainly done for safety reasons, not for fighting against ground loop problems. The mains powered electronics devices which are being repaired are connected to mains power through isolation transformer to make it safer to test them.

A general recommended paractice for electronics device repair shops in Finland is to use at least two mains isolation transformers in their repair environent. The first one is an isolation transformer whcih isolated all the test equipments (like oscolloscope) from the mains ground and the other one is the one used for isolating the equipment which is being tested or repaired. This arrangement is very good on both safety and noise reasons.

Sometimes isolation transformers are also used to fight agains noise problems, but generally they are used for safety reasons in electronics laboratories.

Mains isolation transformer does not always help in groundloop problems

It is sometimes suggested that an isolation transformer might be used to solve the ground loop problem. It is true, that somethimes a mains isolation transformer does help, but not always.

The reson that mains isolation transformer will not always work is that there is quite a variety of mains isoltaion transformers designed for different used. Those mains isolation transformers all isolate the main live ant neutral wires, but vary what they do ti the ground connections. Some transformers cut the ground connection to mains ground, some other doesen't.

For example international office product safety regulations including IEC 950 and UL 1950 require that an isolation transformer is only allowed to isolate the hot and neutral wires; the grounding wire must be passed straight through. Since the computer circuits including data communication circuits are connected to the grounding wire and not the neutral wire, the isolation transformer or any power conditioner or UPS with an isolation transformer has absolutely no affect on computer grounding problems.

For laboratory use there is available special isolation transformers which also isolate the ground connection (at least in Finland). This tranformer provides complete isolation of live and neutral wires. The output can be non-grounded electrical output because the tranformer provides adequate isolation. Remeber that you are generally allowed to connect only one equipment to this kind of safety isolation transformer. If you connect more that one equipments to safety isolation transformer, you can create a potential hazard. This kind of transformer is good laboratory environment, but it might not considered safe or not meeting the electric code outside laboratory.

Tomi Engdahl <[email protected]>