230vac to 16.5vac Stepdown Transformer-Fuse requirement

retepnosnikta

Just had the above made made up to connect to an alarm control panel The input to the panel is 16.5vac.There is no fuse on the control panel.There is no fusing on either the primary or the secondary winding of the transformer.The spec for the transformer as follows

Primary winding-
kVA

---

0.05
Voltage--230
Amps----0.2
Rate

---

Cont
Phase 1-50

Secondary winding
Volts----16.5
Amps----3.0

What I'm trying to work out is where should I fuse the transformer, at the primary or the secondary winding and what rating of fuse should I use. I understand there is a difference between an ac and a dc fuse so I'm looking at ac fusing here

Bruthal

The fuse, if needed, would be on the secondary side. If you imagine a short causing a 6 amp current from the secondary side, the primary would be 0.4 amps. Not very practical to fuse for that increase on the primary. The secondary will have a much wider magnitude variation even though the percentage variation is the same on both sides, so is more practical to fuse.

An AC fuse would likely be de-rated when used on DC, since AC has a zero current point regularly, helping blown fuses interrupt the current. So a DC fuse will work in an AC circuit better than an AC one in a DC circuit.

Up to a 3 amp fuse might be ok. Or smaller depending on the load of the panel.

Joe1919

Many transformers have a thermal fuse winding that melts at 125 degrees and protects the transformer in the event of short circuit windings etc by melting and disconnection. This is often just a few inches of soft tin type wire wound on the primary sometimes going to connection and may not be noticed.

Since there is no mention of this in the spec., I would presume that none is fitted and therefore (for fire safety reasons), I would fit transformer into metal box and fit a 250 v 200mA slow blow fuse in holder or similar protection on primary circuit.

retepnosnikta

Joe1919 wrote: »

Many transformers have a thermal fuse winding that melts at 125 degrees and protects the transformer in the event of short circuit windings etc by melting and disconnection. This is often just a few inches of soft tin type wire wound on the primary sometimes going to connection and may not be noticed.

Since there is no mention of this in the spec., I would presume that none is fitted and therefore (for fire safety reasons), I would fit transformer into metal box and fit a 250 v 200mA slow blow fuse in holder or similar protection on primary circuit.

So Joe you reckon a 0.2 amp fuse on the primary winding. What bothered my was there was no fuse protection on the circuit board so the secondary winding is pushing out 16.5 vac directly connected to the circuit board with no form of protection in the event of an overload.

In fairness most of the power supplies I've dealt with in the past have been of the 220vac to 12vdc variety but this is the first time I've connected a 220vac to 16vac directly to a circuit board. I just want to ensure I've protection both at the mains plug and the circuit.

Joe1919

retepnosnikta wrote: »

So Joe you reckon a 0.2 amp fuse on the primary winding. What bothered my was there was no fuse protection on the circuit board so the secondary winding is pushing out 16.5 vac directly connected to the circuit board with no form of protection in the event of an overload.

In fairness most of the power supplies I've dealt with in the past have been of the 220vac to 12vdc variety but this is the first time I've connected a 220vac to 16vac directly to a circuit board. I just want to ensure I've protection both at the mains plug and the circuit.

Leave the 3 amp in plug or switch for additional protection. It might be no harm to measure the normal current to the transformer and then also to measure the maximum current say when for an alarm unit when all bells are ringing or say, in the case of a unit with a battery, after a power interruption when power unit is both recharging battery and powering unit and possibly bells etc.
For reliability (and to prevent fuse fatigue), especially with a unit that is permanently running, the fuse (even a slow blow) and the power unit itself should always be rated comfortably above the maximum current and well above the normal current.

retepnosnikta

Joe1919 wrote: »

Leave the 3 amp in plug or switch for additional protection. It might be no harm to measure the normal current to the transformer and then also to measure the maximum current say when for an alarm unit when all bells are ringing or say, in the case of a unit with a battery, after a power interruption when power unit is both recharging battery and powering unit and possibly bells etc.
For reliability (and to prevent fuse fatigue), especially with a unit that is permanently running, the fuse (even a slow blow) and the power unit itself should always be rated comfortably above the maximum current and well above the normal current.

Just to make matters a little clearer. I gave the spec to the assembler and he assembled the unit accordingly. What he didn't include was any instruction on fusing requirements. Personally I wouldn't rely on fusing on the plug at the mains so I'm thinking an inline fuse and holder at the primary winding and possibly an inline arrangement at the secondary winding before I connect up to the 16vac terminals at the control panel.

Obviously there's a lot of sensitive electronics on the control panel and my main aim is to protect them. I know the biggest inrush of current will be when I initially energise the power supply but after that it's down to current fluctuations during normal operating conditions and under alarm conditions as was mentioned above.

2011

For primary & secondary sides of the transformer:

Work out your design current (maximum current drawn ignoring inrush). Select a the next fuse size above this value. Ensure that the cable current carrying capacity is greater than the fuse rating.

retepnosnikta

2011 wrote: »

For primary & secondary sides of the transformer:

Work out your design current (maximum current drawn ignoring inrush). Select a the next fuse size above this value. Ensure that the cable current carrying capacity is greater than the fuse rating.

Thanks for that.

As I said earlier I wouldn't have a lot of technical experience here but I came across this and am a little confused. An AC to AC transformer that doesn't requiring fusing.

http://www.coleparmer.co.uk/assets/catalog_pdfs/1314_PDF_CPUK/TW_0426.pdf

retepnosnikta

retepnosnikta wrote: »

Thanks for that.

As I said earlier I wouldn't have a lot of technical experience here but I came across this and am a little confused. An AC to AC transformer that doesn't requiring fusing.

http://www.coleparmer.co.uk/assets/catalog_pdfs/1314_PDF_CPUK/TW_0426.pdf

Also came across this interesting article

http://www.edn.com/design/power-management/4418393/AC-power-your-circuit-without-a-transformer

2011

retepnosnikta wrote: »

An AC to AC transformer that doesn't requiring fusing.

Not trying to be pedantic but transformers are always AC to AC.


The link you provided is for a transformer that has thermal protection built in. Most likely this is the reason that the manufacturer decided that no fuse is required.

retepnosnikta

2011 wrote: »

Not trying to be pedantic but transformers are always AC to AC.


The link you provided is for a transformer that has thermal protection built in. Most likely this is the reason that the manufacturer decided that no fuse is required.

Thanks again. I'll try and contact the assembler again and see if they can clarify the situation regarding fuse requirements at the primary and secondary winding.

As I stated I gave them the spec of what was required and was a little surprised to see no information regarding the placement of fuses. I wired in the stepdown power supply and it works fine but I wouldn't leave it operating 24/7 until I'm certain of the fuse situation. The panel will draw maximum of 1 amp under load.