Page 124 - Louth Transformer Catalogue

LT DATA

PROTECTIVE DEVICES

PROTECTION OF THE SUPPLY (INPUT)
When a transformer is energised, an initial surge current is produced. A protective device must not break under
the initial surge current. The protection of the supply circuit must only take into account the risk of fault current as
the protection against overload must be situated on the secondary circuit. Below is a list of appropriate protection
based on primary voltage and VA rating.
230v 415v
VA
aM Fuse Type C M.C.B. aM Fuse Type C M.C.B.
50 1 1 1 1
100 1 3 1 2
150 1 6 1 2
200 1 6 1 3
250 2 6 1 3
300 2 10 2 6
400 4 10 2 6
500 6 16 4 10
750 8 16 6 10
1000 10 20 6 16
1250 10 25 8 20
1500 10 32 10 20
2000 12 40 10 25
2500 16 40 10 32
3000 16 50 12 40
3500 20 50 16 40
4000 20 63 16 50
5000 25 80 20 63
OVERLOAD PROTECTION (OUTPUT)

Clause 14 of BS 3535: Part 1: 1990 specifies that a transformer should not become unsafe in the event of a short-
circuit or overload. The table below shows a suggested rating for a protective device based on secondary voltage
and VA rating.

Secondary Voltage
VA
12v - 0 - 12v 24v - 0 - 24v 55v - 0 - 55v 240v
50 2 1 0.5 0.2
100 4 2 1 0.5
150 6 4 2 1
200 8 4 2 1
250 10 6 4 2
300 12 6 4 2
400 16 8 4 2
500 20 10 6 2
750 16 7 4
1000 20 10 4
1250 12 6
1500 16 6
2000 20 8
2500 25 10
3000 25
3500 32
4000 38
5000 50
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