Pertanyaan ini muncul saat ingin melakukan design untuk
trafo power 480V/400V, 50Hz, Dyn-11 dengan kapasitas 150KVA yang akan ditempatkan
disebuah portable container.
Dalam NEC (NFPA 70) artikel 450.21 dijelaskan demikian.
NEC
450.21 Dry-Type Transformer
Installed Indoors.
(A) Not over 112½ KVA. Dry-type transformer installed
indoors and rated 112½ KVA or less shall have a separation of at least 300 mm
(12 in.) from combustible material unless separated from the combustible
material by a fire resistant, heat-insulated barrier.
Exception: this rule shall not apply to transformer rated for 600 volts, nominal, or less that are completely enclosed, with or without ventilating openings.
(B) Over 112½ KVA. Individual dry-type
transformers of more than 112½ KVA rating shall be installed in a transformer
room of fire resistant construction. Unless specified otherwise in this
article. The term fire resistant means a construction having a minimum fire
rating of 1 hour.
Exception No. 1: Transformer
with class 155 or higher insulation systems and separated from combustible
material by a fire-resistant, heat insulating barrier or by not less than 1.83
m (6 ft) horizontally and 3.7 m (12 ft) vertically.
Exception No. 2: Transformer
with class 155 or higher insulation systems and completely enclosed except for
ventilating openings.
(C) Over 35,000 Volts. Dry-type transformer rated over
35,000 volts shall be installed in a vault complying with part III of this
article.
Mengacu standard tersebut diatas terutama point (B)
Exception No. 2 disebutkan pengecualian untuk trafo dengan class 155 atau
diatasnya. Lalu muncul pertanyaan kembali apakah yang dimaksud dengan Class
155?
Dalam sebuah artikel dijelaskan demikian.
The insulation system of a transformer is rated in
degrees Celsius at its maximum temperature rating:
The class number
= the maximum °C of the transformer insulation
NEMA (National Manufacturer’s Association) has the
following thermal or insulation classification as regards transformers (dry
type):
Note that a transformer with a Class 220 insulation
system can be designed for a maximum temperature rise that is lower than the
standard 150°C .
It can be designed for either 125 or 80°C rise. Also, a Class 180 insulation
transformer can be designed with 80°C rise. Class 155 and 105 transformers are not
typically designed for other than their standard temperature rise.
Maka untuk design yang optimum dengan mempertimbangkan NEC
artikel 450.21 saya memutuskan untuk menggunakan trafo kering dengan insulation
class F, cooling AN dan IP protection 21.
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