Page 497 - Industrial Power Engineering and Applications Handbook

Instrument and control transformers: application and selection 15/471

(a) LT epoxy resin cast

(b) LT fibreglass taped (c) taped LT PVC (d) Ground leakage relay with CBCT
Figure 15.13 Ring type CTs for measuring or protection (Courtesy: Prayog Electricals)

provide greater mechanical strength and a better (i) Rated secondary current
insulation system. They are more suitable for humid,
contaminated and corrosive atmospheres and for all This will be 1,2 or 5 A, 1 and 5 A being more common.
HT systems. They are mechanically strong and can All measuring and protection devices are also
bear shocks and impacts. manufactured for these ratings only as standard practice.
A 1 A secondary is not recommended in higher ratios
due to increased induced voltage on the secondary side
Note during an accidental open circuit on load. It may damage
All HT CTs are normally manufactured with epoxy resin cast.
the inter-turn insulation or cause a flashover, besides
being dangerous to nearby components or a human body,
Fibreglass tape (Figure 15.13(b)). To make it more if in contact. This we have illustrated in Example 15.8,
compact. under section 15.9.
Polypropylene
(ii) Rated output
15.6.1 General specifications and design
considerations for current transformers The standardVAvalues may be one of2.5,5,7.5, 10, 15
and 30 generally, depending upon the application, although
The rated frequency, insulation systems and the a value beyond 30 VA is also acceptable.
requirement of creepage distances will generally remain
the same as for a voltage transformer (Section 15.4.1). (iii) Rated burden
For the remaining parameters, the following may be
noted. This is the value of the impedance of the secondary