Current Transformers
Circular moulded and resin-cast current transformers for accurate current sensing and metering.
About Current Transformers
A current transformer (CT) steps a large feeder current down to a standard 5 A or 1 A secondary, so meters and relays can measure it safely. We supply low-tension CTs from Selec and Procom for switchboards, MCCs, APFC panels and DG panels.
Procom's LT range runs up to 6000/5 A in rectangular resin-cast form. It covers moulded rectangular and round CTs, resin-cast ring and rectangular types, wound-primary CTs for low currents, 5P10 protection CTs and core-balance CTs (CBCTs) for earth-leakage relays. Selec adds tape-wound, busbar-type and split-core CTs. Split-core types clamp around an existing conductor without disconnecting it, which makes retrofit metering simple.
The ratio is only half the selection. A metering CT needs the right accuracy class, and its VA burden must cover the meter plus the resistance of the secondary leads. A protection CT needs a protection class such as 5P10 so it stays accurate at fault current. We check all three against the meter, the relay and the lead length.
Key specifications
| CT types | Moulded (ring / rectangular), resin-cast ring and rectangular, wound-primary, tape-wound, busbar, split-core, CBCT |
|---|---|
| Primary ratings | 5/5 A wound-primary up to 6000/5 A resin-cast (Procom); split-core 100–6000 A (Selec) |
| Secondary | 5 A standard; 1 A on request |
| Metering accuracy | Class 0.5 and Class 1 (Procom); Class 0.5 to 5 across Selec types |
| Protection class | 5P10 resin-cast; 5P5 and PS class listed (Procom) |
| Rated burden | 2.5 VA to 20 VA, depending on ratio and type |
| CBCT | 1000-turn core-balance CTs, 37.5 mm to 200 mm window; larger resin-cast CBCTs (Procom) |
| Moulded housing (Procom PRO) | Nylon UL 94-V2, IP20, finger-safe terminal cover |
| Selec busbar CT approvals | CE, UL, ASTA |
Features & benefits
- Window sizes and busbar slots to suit standard busbar sections
- Split-core CTs for retrofit metering without a shutdown on the conductor
- Wound-primary CTs give usable accuracy at low currents (down to 5 A)
- Dual terminals for a CT shorting link on Procom moulded CTs
- Protection-class CTs for overcurrent and earth-fault relays
- CBCTs matched to earth-leakage relays
Where it’s used
- LT switchboards and PCC incomers
- MCC feeders and motor protection
- APFC panels
- DG and AMF panels
- Energy metering and sub-metering
- Earth-leakage protection
Standards & compliance
Current Transformers — FAQs
How do I size a current transformer?
Start from the feeder's full-load current and pick the next standard ratio up. Then check the burden: the CT's rated VA must cover the connected meter or relay plus the resistance of the wire run to it.
What is the difference between a metering CT and a protection CT?
A metering CT, such as Class 0.5 or 1, is accurate around normal load current. A protection CT, such as 5P10, stays within its error limit up to many times rated current, so a relay can see a fault correctly. Use the class that matches the device connected to the CT.
What is a CBCT used for?
A core-balance CT surrounds all the phase conductors, plus neutral where present, of a feeder. It measures the residual current that leaks to earth and feeds an earth-leakage or earth-fault relay.
Can I fit a CT without disconnecting the cable?
Yes, with a split-core CT. Selec's split-core range opens around the existing conductor and is offered from 100 A to 6000 A.
