hb039 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

All Advance CVT’s have automatic overload characteristics. In normal situations the output current will limit at about 2 times rated current. 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.

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

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.

part of the AEL technical handbook return to hb000 Handbook Index

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