Current transformers explained, and changing a CT ratio

A current transformer measures the current flowing through a cable and scales it down to a safe level for the meter to read. The ratio is printed on its body. Get the ratio wrong on the platform and every reading is scaled wrong; fit one rated below the panel and you risk damaging it.

Reading a current transformer

The ratio is printed on the transformer itself, for example 100/5A or 2000/5A. The first number is the primary current it is rated for, the second is what it outputs.

Never fit one rated below the panel or breaker rating. A CT smaller than the load it sits on can fail. Confirm the breaker rating, then confirm the CT rating, then install.

Existing CTs rarely transfer between boards. If a meter is being moved from a 400 amp board to a 100 amp board, the CTs from the old site will not be suitable. Expect new ones to be quoted with any relocation. See Moving a meter or cancelling monitoring.

Changing the ratio on the platform

If the ratio configured on the platform does not match what is physically fitted, every scaled reading is wrong by that factor. Email support@augos.io with the point path and the correct ratio.

Two things to know:

1. Historical data does change, but only after a recache. Amending the ratio fixes readings going forward immediately; the historical record reflects the new ratio only once the data has been recached. Ask for the recache explicitly and support will confirm when it is done.
2. Recaching is strenuous. Confirm the correct ratio once rather than iterating. Repeated changes and recaches on the same point cause problems on the platform and Augos will ask why it keeps changing.

Where the meter or an associated device also has a ratio configured on it, that has to be changed on the device as well as on the platform. Both sides need to agree.

Polarity

A CT fitted the wrong way round makes its phase read negative. See Negative readings.

Quoting a replacement

Augos needs two photographs:

1. The existing DB amperage, so the correct type is quoted.
2. The intended installation position, so the correct physical size is specified. Ratio is not the only constraint; the CT has to physically fit around the conductor in the space available.

Still stuck

Email support@augos.io with the point path, the ratio printed on the CT, the breaker rating, and photographs of the CT and the installation position.