Dry Type Transformers: Conductor and Protection Selection – Code File, December 2021
November 28, 2021 – Why are the criteria for installing a transformer so complex? Why can’t it be simpler?
Due to all the rules of the CE Code, there is always other things that come into play. To be fair, these rules try to be flexible enough to allow the installation to go unhindered, with the end result being a functional and safe XFMR installation.
For the purposes of this column, I’ll use examples of one of our most common installation voltages – 600V / 208V – and refer to a single dry-type XFMR with a turn ratio of 2. , 88. In this context, I will discuss the rules of the CE Code and how they interact with each other to ensure a safe installation.
The first rule we need to look at is 26-254 “Over Current Protection”. Subrule 1 states that as long as the O / C protection does not exceed 125% of the rated primary overcurrent and the secondary conductors are rated for at least 125% of the secondary current, the secondary O / C protection can be omitted. .
However, if there is to be secondary O / C protection set at no more than 125% of secondary and the primary is powered by a circuit protected at no more than 300% of rated primary current, then the primary does not require a individual O / C device.
But what happens when the rating of the O / C device does not correspond to 125% of the primary ratings of the transformer?
The CE code asks us to move to the next highest device. This does not mean, however, that you no longer need to follow the other applicable rules. So, if we increase the primary O / C device to keep the transformer running, then we need to make sure that the secondary remains protected at no more than 125% of the rated secondary.
Note that sub-rule 3 only provides an exemption to primary O / C protection, not secondary protection.
This scenario is covered in more detail in Driver Rule 26-256 and references Protection Rules 14-100 and 14-104, so let’s take a look.
Rule 26-256 tells us that conductors supplying the primary and secondary of an XFMR must have a current rating of at least 125% of the primary or secondary rated current. The conductors can also be sized according to the load demand; however, this will now require that the primary and secondary conductors be protected in accordance with the rules in section 14.
This requirement may also involve Table 13 if the calculated load is also known.
Earlier I mentioned the possibility of increasing the primary O / C device beyond the 125% where the value did not correspond to the 125% of the rated primary current. We know that the secondary protection should then not be higher than 125% of the rated secondary current.
This subrule covers 14-100, which means that the primary conductors must always be sized to be protected by the increased O / C protection, and the secondary conductors must also be protected by an O / C device. Now, however, we also need to add the length of a primary circuit conductor more the length of a secondary circuit conductor, and this must not exceed 7.5 m in total length.
When determining the installation and protection of a dry-type transformer, ask yourself: what am I trying to achieve? How to best achieve this while controlling costs and without violating the CE Code?
David Pilon is Director, Electrical Inspections, Technical Safety Authority, Saskatchewan (TSASK). He is also vice-chair of the Committee of Certified Electrical Inspectors of Canada (CCEI) of the International Association of Electrical Inspectors (IAEI), Canadian Section. He can be reached at [email protected]
This feature, along with other interesting content, appears in the December 2021 edition of Electrical Business magazine. Even more back issues can be found in our digital archive.