Isolation Transformer Earthing: What to Bond and When

The transformer’s metal casing and other exposed metal parts must be connected to the protective ground according to the specifications; as for whether the secondary winding is connected to ground, that is another matter and must be decided separately. Don’t think that just because it’s called “isolation transformer”, you can save it by protecting the ground; don’t think that the first and second sides are separate, so the second side must be suspended. The ultimate solution depends entirely on what load you want to carry and how the system intends to detect and remove faults.
So, the three words “isolated” printed on the drawing are at most the beginning. Before signing and confirming, you have to dig out the grounding point of the housing and the grounding reference point on the secondary side (if any), and you have to deduce it in your mind: in case of leakage, which way will the fault current run back to the power supply?

Which areas must be protected and grounded?

The outer shell, frame and accessories that can be touched by hand must be protected and grounded according to the approved drawings. The downstream equipment housing, metal wire slots, and bridges also make sense. The wires on the first and second sides have indeed separated, but this does not mean that if the insulation breaks or the metal parts become live, people will not need protection if they touch them.
Note that the core, shield, and secondary side neutral point are three completely different types of terminals. Some transformers come with dedicated core grounding points or electrostatically shielded terminals. Do not use them as spare protective ground (PE) terminals for random connections. Before you start, look at the specific terminal drawings. Also, don’t just think of it as “isolation transformer” and make up for the fact that it has its own shielding layer.
Also, the so-called “independent ground (Isolated ground)” is by no means a way to leave you ungrounded. If the design requires a separate protection line for sensitive equipment, that line will have its own specific destination throughout the protection system. If you try to save yourself the trouble of breaking the ground stakes yourself, or simply disconnect the grounding of the casing, you will not achieve the same protection effect.

When should the secondary side be grounded?

If the downstream power distribution system needs a clear grounding reference point, then the secondary side must be grounded. The drawing must clearly show where this point is connected and how the protection switch will act in the event of a malfunction. Remember to check the transformer cabinet, the first downstream switchboard, and the electrical equipment to see if there are any short zero-ground connections that others have overconnected. This extra parallel path is very easy to miss when the load voltage is normal.
The secondary side can only remain suspended if the entire system is designed to be ungrounded. In IT-type (ungrounded) systems, exposed metal parts must also be protected and grounded according to regulations. When the first insulation failure occurs, an alarm must be issued and someone must investigate it. The design must also consider what to do if the first failure is not repaired and a second failure occurs.“ Suspension operation” is a serious protection decision and is definitely not an excuse to cut corners.
In addition, for a specific load “electrical isolation”, this is different from the IT power distribution system of the entire plant. A test bench dedicated to a specific device has different system boundaries than the entire factory network. If you add meters, communication lines, or other loads, this boundary changes.

What factors can disrupt the pre-set grounding architecture?

The connecting wires outside the transformer cabinet can change the state of the suspension circuit in minutes. A ground-connected test instrument, a shielded signal line, an EMC filter, or a connected machine can create a circuit that is invisible on purely electrical drawings. Before determining the secondary side “which has absolutely no ground reference point”, go through all of these interfaces.
Measuring the voltage between the suspended wire and ground using a multimeter does not directly indicate anything. The capacitive coupling effect, combined with the characteristics of the multimeter itself, makes it easy to measure a false voltage, which needs to be further investigated. Write down what equipment was connected when measuring the voltage and retest it using on-site approved testing methods; do not rashly change the ground wire just because you see a voltage reading.
When it comes to UPS projects, a grounding system diagram is definitely not enough. You need to mark the neutral line direction, transformer position and protective grounding conditions under normal mains operation, battery power, static bypass and maintenance bypass. Looking at the perfectly correct grounding architecture in one mode, cutting into another mode may completely change it.

What must be verified during field debugging?

The task of on-site debugging is to prove that the actual work done is exactly the same as the approved drawings. Before power is supplied, check the transformer nameplate and terminal markings, check whether the protective ground wire is connected properly, and pull out all the wires that are inexplicably connected to the upper secondary side and the ground. Test methods and conformity standards must follow product and field specifications; it is unrealistic to expect to use a uniform “ground resistance value” to cover all isolation transformers.
The operation of the protection system must be checked and recorded. If it is the secondary side of the ground, pair the switches and set values installed on site with the fault assessment report. If it is an IT system, test the alarm status of the insulation monitor and determine who is responsible for handling the alarm. If UPS is involved, the handover record must include the test results in the relevant switching mode.