Instrument transformers are the least glamorous item on a panel schedule and the one most often specified by copying the last project. That is fine until the day a protection CT saturates during a fault and the relay does not see it.
Metering and protection are different jobs
A metering CT must be accurate at normal load and is allowed — indeed expected — to saturate under fault current, so the instruments behind it are protected. A protection CT must stay linear well beyond rated current so the relay sees the fault faithfully.
Using one for the other fails in both directions: a metering CT in a protection circuit blinds the relay, and a protection CT in a metering circuit gives you a bill you cannot trust.
| Metering CT | Protection CT | |
|---|---|---|
| Typical class | 0.2S, 0.5, 1.0 | 5P10, 5P20, 10P10 |
| Accurate at | Rated current | Up to 10–20× rated current |
| Behaviour in a fault | Saturates, protecting instruments | Stays linear so the relay operates |
| Specified by | Accuracy class and burden | Class, burden and accuracy limit factor |
Accuracy class, plainly
Class 0.5 means the ratio error stays within 0.5% at rated current. Class 0.2S is tighter still and holds accuracy down to 1% of rated current, which is why utilities specify it for revenue metering — a plant that idles overnight is still being billed.
For protection, 5P20 reads as: 5% composite error, up to twenty times rated current. The number after the P is the accuracy limit factor, and it is the number that matters for relay coordination.
Burden is not optional
Burden is the load the CT drives, in VA, and it includes the relay or meter plus the resistance of the cable run. A 10 VA CT feeding a 5 VA relay through a long run of thin cable can easily be over-burdened, and an over-burdened CT does not meet its class.
- Add up the connected device burden in VA at the rated secondary current
- Add the cable burden — two × run length × resistance per metre × I²
- Choose a CT rated above that total, with margin, not exactly at it
- Prefer 1 A secondaries for long runs; the cable burden falls by a factor of 25
Knee point voltage, for differential protection
Where CTs feed a differential or restricted earth fault scheme, the knee point voltage matters more than the class. Both CTs in the pair must be closely matched, or the scheme will see a difference that is not there and trip on through-faults.
Potential transformers
The same logic applies. Class 0.2 or 0.5 for metering, 3P or 6P for protection. The specific point to watch on PTs is ferro-resonance in unearthed systems — a phenomenon that destroys PTs quietly and repeatedly until someone fits a damping resistor across the open delta.
We cast our own CTs and PTs in-house up to 33 kV, which means we can match a pair for a differential scheme rather than shipping whatever is in stock.
What to tell your supplier
- System voltage and whether the neutral is earthed
- Ratio, and whether you need dual ratio or multiple cores
- What each core feeds — meter, relay, or both
- Accuracy class and burden for each core separately
- Short-time thermal and dynamic current ratings of the system
- Indoor or outdoor, and the pollution level if outdoor