Every current transformer on our datasheets carries an accuracy class code — 0.2S, 0.5S, 5P10, 10P10, and combinations of these on dual-core units. It looks like a minor spec, but it is the single biggest factor in whether a CT is right for a revenue meter, a protection relay, or both.
Two different jobs, two different classes
A metering core has to stay accurate at normal load, because that is where billing happens. A protection core has to stay linear at enormous fault current, because that is when a relay has to trip correctly. No single core does both well, which is why our combined metering boxes and higher-current CTs are commonly built with two secondary cores rated differently — for example 0.2S for the meter and 10P10 for the relay on the same primary winding.
Metering classes: 0.2, 0.5, and the "S" suffix
The number — 0.2, 0.5, 1 — is the maximum permissible ratio error, in percent, at rated current. A 0.2-class CT is accurate to within 0.2% at rated current; a 0.5-class CT to within 0.5%. The "S" suffix (0.2S, 0.5S) means the accuracy is guaranteed down to 1% of rated current as well as up to 120% of it — important for sites where load varies widely, since an ordinary 0.2 or 0.5 class CT is only guaranteed accurate across a narrower band around rated current.
Protection classes: 5P and 10P
A "P" class code — 5P10, 5P20, 10P10 — describes behaviour during a fault, not at normal load. The first number (5 or 10) is the maximum composite error at the accuracy limit current, in percent. The second number is the accuracy limit factor (ALF): 5P10 means the CT stays within 5% error all the way up to 10 times rated current — the working range a protection relay actually needs to see to trip correctly during a fault.
Reading a combined rating
On our JLSGWV/JLSGWY metering box datasheets you will see accuracy combinations written as "0.2S/10P10" or "0.5S/10P10" against a single secondary output. That means one secondary output is guaranteed to both classes simultaneously up to the stated limits — a simpler, lower-cost option than a fully separate dual-core CT where the application allows it. Where metering and protection genuinely need to be electrically independent, specify a dual-core unit instead.
How to choose the accuracy class for your application
Start from the function, not from the price list. For revenue metering and billing, utilities normally specify a 0.2S or 0.5S class core: the "S" suffix is the important part, because it guarantees accuracy all the way down to 1% of rated current. That matters on distribution transformers that run lightly loaded at night. For general monitoring, panel indication and industrial energy management, a standard 0.5 or 1.0 class is usually enough. For protection cores, choose 5P or 10P according to the relay setting and the through-fault current the core must pass without saturating.
A single current transformer frequently carries both jobs — one metering core and one protection core in the same body, each with its own accuracy class, ratio and rated burden printed on the nameplate. Specify each core separately, exactly as it will be loaded in service.
Accuracy class, burden and ratio are chosen together
A class code is only a promise within limits. A 0.2S core delivers its rated error only when the connected burden does not exceed the rated burden on the nameplate (for example 10 VA or 30 VA) and when the primary current stays inside the guaranteed range. Choose the ratio first — typically 1.2 to 1.5 times the maximum continuous load — then check the burden, then confirm the class is still achievable at that burden. A complete specification reads like this: "200/5 A, 0.2S, 30 VA, 12 kV insulation".
Frequently asked questions
What does the "S" suffix mean on a CT accuracy class such as 0.2S?
The "S" class (special) guarantees the accuracy limits from 1% up to 120% of rated current, instead of the standard 5% lower limit. It is specified for billing meters that must stay accurate at very light load.
What is the difference between 0.5 and 0.5S?
Both allow a 0.5% current error at rated values, but the 0.5S variant extends the guarantee down to 1% of rated current. For sites with significant low-load operation, prefer the S version.
Which accuracy class is required for revenue metering?
Most utilities specify 0.2S or 0.5S for fiscal metering points, depending on the tariff and the site importance. Confirm the local utility regulation before ordering; the difference is small in price but significant in billing disputes.
What does 10P mean on a protection CT?
The "P" identifies a protection class: 10P means the composite error is guaranteed to stay within 10% at a stated multiple of rated current (for example 10P10 = at 10 times rated current). 5P is the tighter protection class.
Can one CT serve metering and protection at the same time?
Yes — with dual cores inside the same transformer. Each core carries its own accuracy class and rated burden, so the metering circuit and the relay circuit never load each other.