Which Fuse for a Constant Voltage Transformer? Fusing and start up surges

The switch on current surge for a Constant Voltage Transformer ( CVT) consists of two components. One of these is fixed at about 8 times running current for 5 - 10 mSec. Superimposed on this will be a 'spike' which will be dependent on where in the mains cycle the transformer was last turned off and where in the cycle it is turned back on. The spike will be less than 1 mSec and varies from zero to 25 times running current if the supply is very 'stiff'. The input surge current at switch on is not significantly affected by the output load condition. The surge current will be substantially proportional to applied line voltage. Our normal circuit breaker recommendation is to fit a 'Type 4 ', 'TypeD' or 'Curve 66' (i.e. motor rated) unit which should function without nuisance tripping. Fuse earth loop impedance and cable size suggestions are shown.

Fuse ratings

Fuses known as 'motor circuit fuse links' in BS 88 / IEC629 are ideal for CVTs. The GEC type 'T' H.R.C. are used in the table and European types should be the type called 'aM' or if available type 'gTr'. We suggest that installers use a wall-mounting switch fuse arrangement or 'red-spot' fittings. The table shows the MINIMUM fuse that can be used in normal installations, if using a breaker the choose the next size LARGER in your breaker range. Attention must be given to special cases where input voltage limits may go below the -20% figures used in the table. Cable sizes are also the minimum recommended by GEC for use with each fuse rating. The fuses will protect a PVC cable according to rule 433-2 for 'open conditions'. Earth loop impedance is for SAFETY considerations and does not reflect the needs of a clean earth. Protection against electric shock is provided by these fuses when correctly installed. Part numbers are based on GEC published data and some so-called direct equivalents may not be suitable. Discrimination must be proven under the requirements of the current edition of the local permanent wiring regulations BS 76711992 - it remains the responsibility of the installer to ensure that the supply is protected.

Output fuses

The inherent short circuit protection of a CVT makes output circuit fault protection problematic. In particular normal models for discrimination that apply well to conventional transformers do not apply to CVTs. In a circuit containing a conventional transformer, the expected short circuit current can be calculated by equating the transformer to a large series impedance. The direct linkage between primary and secondary circuits allows the typically large input short circuit current to blow fuses on the output with only slight mitigation. The CVTs galvanic isolation feature however means the complete short circuit of the output is in no way linked to the input, as such the input fuse will not blow and the only energy available to blow the output fuse is that available in the saturated output circuit.

Typical short circuit currents for CVTs are 150-200% of rated current.

Real Life Examples

Tests performed on an AGT 3000J (3000VA) CVT with an output rating of 230V 13A, found the short circuit current to be 19A continuous with no adverse affects on the CVT or the input fuse. Clearly this transformer will never blow a 20A output fuse and will take a very long time to blow a 16A version, In fact because the output characteristic of the current is dependent on when in the sine wave the short circuit is applied in 1/3 of tests even a 6A fuse didn't blow instantly.

The output voltage will collapse to near zero dependent on the fault impedance. It is impractical to provide a fuse that will remain intact for normal full load use and definitely open under fault conditions since the CVT does not and cannot supply enough energy. We therefore recommend no output fusing be used except where other high power conductors could become connected under fault conditions to the output circuit of the CVT or the fuse required is much smaller than the output capacity of the unit. If it is essential to protect the output we have had good success with thermal circuit breakers.

Output protection using thermal breakers

Since the CVT is capable of developing a steady current when marginally overloaded and exhibiting an output voltage which is slightly lower than normal it is practical to protect the load wiring with a thermal breaker. The breaker should be chosen so that under all normal operating conditions it will not see it's rating. If a fault occurs causing excess current to flow then the breaker will eventually open - even if the overload is only 105-110% of the breaker rating. The breaker will open quite quickly in the event of a short circuit in the secondary load wiring.

In circumstances where specific fuse blowing or discrimination is required and a CVT is fitted Advance can recommend a VERY OVERSIZED CVT based upon the required output fuse rating

115Vac systems

VA rating Amps @88Vac BS88 Fuse A IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms VA rating Amps @88Vac BS88 Fuse IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms
100 1.5 5 5aM 1 13 5000 65 100M125 100aM A4 16 0.45
200 2.7 10 10aM 1 7.7 6000 78 100M160 100aM A4 16 0.45
250 3.4 10 10aM 1 4.4 7000 88 100M200 100aM A4 25 0.45
300 3.9 13 13aM 1 4.4 8000 107 200M250 200aM B2 35 0.19
400 5.0 13 13aM 1 4.4 10000 134 200M250 200aM B2 35 0.19
450 5.7 13 13aM 1 4.4 12500 158 315M355 B3 50 0.11
650 8.7 13 13aM 1 4.4 15000 189 315M355 B3 50 0.11
1000 13 20M25 20aM A1 1.5 3 20000 267 315M355 B3 50 0.11
1500 19 32M50 32aM A2 4 1.8 25000 334 400M450 B4 70 0.096
2100 27 32M63 32aM A2 6 1.8 33000 441 400M450 B4 70 0.096
3000 39 63M80 63aM A3 6 0.86 50000 693 630M670 C2 95 0.054
4000 52 63M100 63aM A3 10 0.86

230Vac systems

VA rating Amps @192Vac BS88 Fuse A IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms VA rating Amps @192Vac BS88 Fuse IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms
100 0.7 3 3aM 1 13 5000 30 32M63 32aM A2 6 1.8
200 1.3 5 5aM 1 13 6000 36 63M80 63aM A3 6 0.86
250 1.6 5 5aM 1 13 7000 41 63M80 63aM A3 6 0.86
300 1.8 5 5aM 1 13 8000 49 63M100 63aM A3 10 0.86
400 2.3 10 10aM 1 7.7 10000 61 100M125 100aM A4 16 0.45
450 2.6 10 10aM 1 7.7 12500 72 100M160 100aM A4 16 0.45
650 4 10 10aM 1 7.7 15000 87 100M200 100aM A4 25 0.45
1000 6 13 13aM 1 4.4 20000 123 200M250 200aM B2 35 0.19
1500 9 13 13aM 1 4.4 25000 153 200M250 200aM B2 35 0.19
2100 12 20M25 20aM A1 1.5 3 33000 202 200M315 200aM B2 35 0.19
3000 18 20M32 20aM A1 2.5 3 50000 318 315M355 B3 50 0.11
4000 24 32M50 32aM A2 4 1.8

400Vac systems

VA rating Amps @332Vac BS88 Fuse IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms VA rating Amps @332Vac BS88 Fuse IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms
100 0.4 3A 3aM - 1 13 5000 17 20M32 20aM A1 2.5 3
200 0.7 3A 3aM - 1 13 6000 21 32M50 32aM A2 4 1.8
250 0.9 3A 3aM - 1 13 7000 23 32M50 32aM A2 4 1.8
300 1.0 3A 3aM - 1 13 8000 28 32M63 32aM A2 6 1.8
400 1.3 5A 5aM - 1 13 10000 35 63M80 63aM A3 6 0.86
450 1.5 5A 5aM - 1 13 12500 42 63M100 63aM A3 10 0.86
650 2.3 10A 10aM - 1 7.7 15000 50 63M100 63aM A3 10 0.86
1000 3.4 10A 10aM - 1 7.7 20000 71 100M125 100aM A4 16 0.45
1500 6.4 13A 13aM - 1 4.4 25000 89 100M160 100aM A4 25 0.45
2100 6.8 13A 13aM - 1 4.4 33000 117 200M250 200aM B2 35 0.19
3000 10 20M25 20aM A1 1.5 3 50000 187 200M315 200aM B2 35 0.19
4000 14 20M25 20aM A1 1.5 3

480Vac systems

VA rating Amps @398Vac BS88 Fuse IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms VA rating Amps @398Vac BS88 Fuse IEC269 Fuse UK Holder Cable sq mm Earth loop Ohms
100 0.3 3A 3aM - 1 13 5000 13 20M25 20aM A1 2.5 3
200 0.5 3A 3aM - 1 13 6000 16 32M50 32aM A2 2.5 1.8
250 0.6 3A 3aM - 1 13 7000 18 20M32 20aM A1 2.5 1.8
300 0.8 3A 3aM - 1 13 8000 21 32M50 32aM A2 4 1.8
400 1.0 3A 3aM - 1 13 10000 26 32M63 32aM A2 6 0.86
450 1.1 3A 3aM - 1 13 12500 32 63M80 63aM A3 6 0.86
650 1.6 5A 5aM - 1 7.7 15000 38 63M80 63aM A3 6 0.86
1000 2.5 10A 10aM - 1 7.7 20000 53 63M100 63aM A3 10 0.45
1500 4.8 10A 10aM - 1 4.4 25000 67 100M125 100aM A4 16 0.45
2100 5.1 13A 13aM - 1 4.4 33000 88 100M160 100aM A4 25 0.19
3000 8 20M25 20aM A1 1.5 3 50000 140 200M250 200aM B2 35 0.19
4000 11 20M25 20aM A1 1.5 3