Substation Design | Protection and Control (P&C) | System Protection | Transmission & Distribution (T&D) | Renewable Energy | Leadership | 18+ years in the Power Industry
During a P&C design review, someone asks: Madjer, where exactly should we ground the CT and VT secondary circuits? This question comes up all the time, yet it’s still one of the easiest places to make mistakes that can cause strange readings, blown fuses, or even unsafe voltages at the relay panel. The general rule is simple: ground the CT/VT secondary at one single point, preferably at the first point of application: the relay panel or switchboard. That’s where overvoltages are most likely to appear, and where a solid ground path offers the best protection for personnel and equipment. However, life is rarely that simple in a substation. Some schemes require grounding at another location because of how secondary windings or devices are interconnected. The goal is always to achieve correct equipment performance without creating circulating currents or losing measurement reference. A few typical arrangements clarify how this works in practice: - If you have one CT or VT, ground one end of that secondary winding. - If multiple transformers feed a common circuit, connect the common secondary point of all windings to a single ground. That covers parallel or cross-connected windings, 3 single-phase units connected in wye, or even open-delta and open-wye voltage transformer sets. - When 3 or more CTs or VTs are connected in a way that lacks a shared neutral, choose a point common to most of the circuits and ground it. The key is still one reference, one path. For differential protection, things get more interesting. When several CT sets are interconnected but cannot share a common neutral (ex: delta-connected CTs feeding a diff. relay) ground the neutral associated with the largest group of CTs. That keeps the circuit at a defined potential and avoids parallel return paths. All of this may sound procedural, but there is a reason behind it: multiple grounds create circulating current loops, which distort secondary readings and can lift the entire circuit above ground potential during faults. A single, well-defined ground keeps every CT/VT and relay operating at the same reference and ensures that secondary voltages stay within safe limits. In past experiences, I’ve seen floating CT circuits burn terminal blocks and VTs show 'phantom' readings after an unintended double ground. It’s rarely a design flaw, although it happens sometimes. Wiring oversights or unclear grounding notes on a drawing happen more often. In my opinion, the best reference to always get it right is IEEE Std C57.13.3 Guide for Grounding of Instrument Transformer Secondary Circuits and Cases. ### Share your experience: How does your team define the single-point ground location during design? Do you prefer grounding at the relay panel or at the instrument transformer itself? And have you ever traced a mysterious CT loop only to find two grounds fighting each other? If you found this post valuable, share it with your network: let’s keep our knowledge solidly grounded ⏚ ⏚ ⏚