Transformer installations and Section 26 • Code File, June 2018
July 27, 2018 — For the first time in the CE Code (2018 ed.), a unit substation is defined as an integrated unit consisting of one or more transformers, disconnecting means, overcorrect devices and other associated equipment, each contained in an enclosure designed and constructed to restrict access to live parts (Rule 26-240).
Rule 26-242 goes on to explain the installation and protection requirements for unit substations, depending on their design. Per Rules 26-300 through Rule 26-324, unit substations require fencing (which can be chain-link or wood-slat construction), unless they are already isolated via elevation
(min. height of 3.6 m to the bottom) or suitably housed in protective enclosures. Signage indicating the highest voltage employed is also required, unless there are no exposed energized parts.
The CE Code 2018 absorbs the 2015 Rule 26-246 “Dielectric liquid-filled transformers indoors” into Rule 26-012 “Dielectric liquid-filled equipment indoors”. The only real issue here is the renumbering of the rules.
The 2018 code also includes a new rule on the installation clearances for transformers in Section 2. Rule 2-312 “Transformer working space” was added to clarify that 1 m of clearance is required for transformers larger than 50kVA, but only on the side requiring access to conductor connections. When the transformer has high-side connections (H) on one side and low-side terminations (X) on another, then both sides will require 1 m access to those connections.
These clearances are in addition to those required for ventilation (Rule 26-246). Because the units are often suspended — requiring access by other means — the clearance requirements do not reference “with secure footing”. Rule 26-242 states that clearances for outdoor transformers and unit substations are greater and are, therefore, exempt from Rule 2-312.
Subrule 26-256(4) has been added to address conductor sizes for transformers. It aims to add clarity to the protection requirement, which was a bit unclear when taken in conjunction with the overcorrect protection allowance, which increases the primary OC protection — as per Rule 26-254(3) — to the next higher standard rating. (Under the 2015 code, we would allow the increase on the primary side, and then allow it again on the secondary, which permitted a number of transformers to work outside of their tested parameters.)
Let’s look at a 75 KVA transformer as an example: when we increase the primary FLA (full load amperes) of 72.254A by the allowable 125%, it becomes 90.317A. When increased by the 125%, the secondary FLA of 208.43A becomes 260.54A. When we installed the overcurrent on the primary (oversized, as allowed, in the 2015 code), then we permitted the secondary to be overdriven.
For example, when you installed a primary OC (set at 100A) on this 75KVA transformer, then the next higher standard on the secondary was accepted as 300A. This is an incorrect interpretation of the intention behind this Rule.
According to Subrule 26- 256(4), we still need to ensure both the primary and secondary conductors are protected in accordance with Rules 14-100 and 14-104. So, if we use 100A OC on the primary, then the primary OC of 100A — multiplied by a turns ratio of 2.88 — equals 288A. However, as mentioned above, 125% of the secondary OC cannot exceed 260.54A. Therefore, secondary protection is required to be 250A; otherwise, we still run the risk of overdriving the transformer and overloading the conductors.
Our second option is to install a primary OC protection set as close to the 125% as possible, without exceeding it. Were we to select a primary OC set at 90A, then 90A multiplied by a turns ratio of 2.88 equals 259.2A. This secondary current is less than 125% of the secondary of the transformer, so the secondary OC protection is not required. Instead, the primary OC device protects it — as per Rule 26-254(1) — and the installation meets the intent of both Rules.
This same Rule 26-256(4) affects the wording of Rule 26-250(1) and (2) for power and distribution transformer circuits rated over 750V.
It’s important to note that the intent of the percentage increase was to provide a base increase in protection ampacity and ensure the OC protection fell within the range of the transformer’s nameplate rating and the allowable operating range.
This is why the Rule states the overcurrent protection is to be set at “not more than” 125% or 150% or 300%.The allowance to go beyond the percentage increase is only when standard protection does not fall within that range. Were we to protect below the nameplate rating of the primary, we’d run the risk of choking the transformer.
This article originally appeared in the June 2018 issue of Electrical Business Magazine.
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