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hb039 Fusing and start up surges on Constant Voltage Transformers (CVTs)

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 CVT's. 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 CVT's. 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 CVT's 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 CVT's 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

230Vac systems

400Vac systems

part of the AEL technical handbook return to hb000 Handbook Index

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