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n
Protection of transformers
In accordance with the IEC EN 61558 standard, transformers must
be protected against overload and short-circuit conditions which may
occur during normal operation.
The standard does not specify the type or location of the protective
device
: it is the manufacturer’s responsibility to choose the most
suitable position, either on the primary or secondary side. The rating,
type and location of the protective device are to be indicated on the
device identification plate.
n
Protection of the primary and secondary circuits
General
Circuits must be protected against overload and short-circuit conditions.
Protection against overload is compulsory if the circuit is likely to be
affected by an overload current. This protection can be installed at the
supply source or end of the circuit.
Protection against a short-circuit is compulsory in all installations ;
this protection has to be installed at the supply source of the circuit.
Supply circuits (transformer primary)
The transformer is a device which cannot itself generate overload. Its
supply circuit therefore only requires protection against a short-circuit.
When a transformer is energised, a very high inrush current (around
25 ln)
1
is produced for approximately 10 ms.
When protecting the circuit, both factors must be considered.
Legrand offers three possibilities :
• type aM cartridge fuses,
• type D MCBs (with magnetic tripping set at 12 ln average)
• type C MCBs (with magnetic tripping set at 7 ln average)
Example : control transformer 630 VA - 230/24 V Cat. No. 0442 37
In primary 2·74 A (VA ÷ V)
I inrush at power-up 68·5 A (25 x 2·74 A)
Protection against a short-circuit can be provided by :
• type aM 6 A cartridge fuse
• type D 6 A MCBs
• type C 13 A MCBs - (16 A nearest UK)
n
Primary protection
Minimum protection rating for primary supply of the transformer :
n
Secondary protection
Ratings and types of protective devices to be used for single phase
transformer protection according to VA rating and secondary voltage
1 : In = nominal rating of circuit
230 V single-phase
400 V single-phase
400 V three-phase
Power
aM
C
D
aM
C
D
aM
C
D
(VA)
fuse MCB MCB fuse MCB MCB fuse MCB MCB
40
0·5
1
0·25
1
50
1
1
1
0·5
1
63
1
2
1
0·5
1
100
1
3
1
1
2
1
150
2
6
2
1
3
1
160
2
6
2
1
3
1
1
200
2
6
3
1
3
2
250
2
6
3
1
6
2
1
300
4
10
6
2
6
2
400
4
10
6
2
6
2
2
500
6
16
6
4
10
3
630
6
16
6
4
10
3
2
6
2
750
8
16
10
6
10
6
2
6
3
1 000
10
20
10
6
16
6
4
10
3
1 250
10
25
16
6
20
10
4
10
6
1 500
10
32
16
10
20
10
4
16
6
1 600
10
32
16
10
20
10
6
16
6
2 000
12
40
16
10
25
10
2 500
16
50
20
10
32
16
6
20
10
3 000
16
50
32
12
40
20
3 500
20
50
32
16
40
20
4 000
20
80
32
16
50
20
10
32
16
5 000
25
100
40
16
63
25
12
32
16
I c/c min. =
24
= 44·42 A = Use either :
24
2
x
3·3
+
2 x 0·027 x 15
(
630 100
)
1·5
44·42
= 11·10
→
gG fuse 10 A max.
44·42
= 5·55
→
C type MCB 5 A max.
4
8
s042715
2 x 1·5
2
15 m
Transformer
protection
25 A gG
Circuit protection
Operation
40
2
T
1
T
400 m T
200 m T
50
2
T
2
1
T
1 500 m T
0·5 200 m T
0·2
63
3·15 T
1·6
T
630 m T
315 m T
100
4
gG
4
2
gG
2
1
gG
1
0·5
gG 0·5
150
6
gG
6
4
gG
4
2
gG
2
1
gG 1
160
8
gG
8
4
gG
4
2
gG
2
1
gG 1
200
8
gG
8
4
gG
4
2
gG
2
1
gG 1
250
10
gG
10
6
gG
6
2
gG
2
1
gG 1
300
12
gG
12
6
gG
6
4
gG
4
2
gG 2
400
16
gG
16
8
gG
8
4
gG
4
2
gG 2
500
20
gG
20
10
gG 10
6
gG
6
2
gG 2
630
25
gG
25
12
gG 12
6
gG
6
3
gG 3
750
gG
16
gG 16
8
gG
8
4
gG 4
1 000
40
gG
40
20
gG 20
8
gG
8
4
gG 4
1 250
gG
gG
12
gG
12
gG
1 500
gG
gG
16
gG
13
8
gG 8
1 600
63
gG
63
32
gG 32
16
gG
13
8
gG 8
2 000
gG
gG
20
gG
20
8
gG 8
2 500
100
gG 100
50
gG 50
20
gG
20
10
gG 10
3 000
gG
gG
32
gG
32
gG
3 500
gG
gG
32
gG
32
gG
4 000
gG
gG
32
gG
32
16
gG 16
5 000
gG
gG
50
gG
50
gG
Rated power
24 V
48 V
110 V
230 V
(VA)
Fuse
Curve
Fuse
Curve
Fuse
Curve
Fuse
Curve
IEC/CSA
A Type
C
A Type
C
A Type
C
A Type
C
MCB
MCB
MCB
MCB
n
Protection of the secondary circuit
The secondary circuit must be protected against overload and short-
circuit conditions. For overload, check that the protection rating
chosen is lower than or equal to the transformer secondary current.
For short-circuit, ensure that a short-circuit occurring at the furthest
point of the circuit will trigger the protective device within 5 seconds.
Legrand offers two possibilities :
• type gG cartridge fuses
• type C MCBs (with magnetic tripping set at 7 ln average)
If the transformer only supplies a single circuit, and provided the
calculations show perfect compatibility, transformer protection (if on
secondary) and circuit protection can be one and the same. A single
protective device performs both functions (see table of transformer
protective devices).
If the transformer supplies several circuits, overload and short-circuit
calculations must be performed for each individual circuit. To check
that the device chosen is suitable, an approximate minimum short-
circuit value can be obtained using the following formula.
n
Formula for determining the secondary protection rating
Calculate the minimum short-circuit at the furthest point on the circuit :
I c/c min. =
Us
U
2
s
x
U c/c %
+
2
ρ
l
(
P 100
)
S
Us = transformer secondary voltage
P = transformer power
U c/c % = transformer short-circuit voltage
I = line length in m
S = line cross section in mm
2
ρ
copper = 0·027
Ω
mm
2
/m
Choose the protection rating so as to obtain a cut-off time of
5 seconds max. for a current I c/c defined above :
I c/c min.
type gG fuse : In
≤
4
I c/c min.
type C MCB: In
≤
8
Example : control transformer 630 VA - 230/24 V Cat. No. 0442 37
These values are given for information only for transformers with inrush
currents of around 25 ln.
protection of transformers, primary and secondary circuits