Lighting transformer tripping MCBs
JimM
Further to a previous post I've purchased the transformer in the link below
for my garden lights,
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=380090544021&ssPageName=STRK:MEWNX:IT
However as soon as I turn it on the MCB/RCD trips on my CU :( Originally I
thought the unit was faulty so I got a replacement sent but both units have
the same problem.
I'm running the transformer on a 30A ring main but it is through a 13A plug
is this likely to be the cause of the problem? If not any suggestions of
basics tests I could do to see if it the transformer itself that is at fault
(obviously the unit is sealed so I can't poke around inside)
Many Thanks
Jim
Bob Minchin
be doing anything useful but make sure one side of the output is earthed.
Bob
JimM
"Bob Minchin" <bob.minc...@YOURHATntlworld.com> wrote in message
news:w2iyo.31712$zz4....@newsfe04.ams2...
>>
> Quite likely to be the input filter. Disconnect the earth lead. It won't
> be doing anything useful but make sure one side of the output is earthed.
>
> Bob
Hi Bob
There's no earth lead to disconnect, just live and neutral on the mains side
and presumably 12v and 0v on the output.
Not sure if it makes a difference but when testing I tried with the lights
connected and not connected both with same results
Thanks
Jim
harry
> Hi guys
>
> Further to a previous post I've purchased the transformer in the link below
> for my garden lights,
>
>
> However as soon as I turn it on the MCB/RCD trips on my CU :( Originally I
> thought the unit was faulty so I got a replacement sent but both units have
> the same problem.
>
> I'm running the transformer on a 30A ring main but it is through a 13A plug
> is this likely to be the cause of the problem? If not any suggestions of
> basics tests I could do to see if it the transformer itself that is at fault
> (obviously the unit is sealed so I can't poke around inside)
>
> Many Thanks
>
> Jim
Is there an earth connection to the transformer? if not, the problem
is not related to an earth problem. If it has, then there is something
odd about your transformer. It really needs an insulation test. Check
that the transformer is the correct voltage etc.
Some of these "transformers" are not actually transformers. They are
electronic voltage reduction devices, you should not be using them on
pond lights as they don't isolate the mains from the lights, hence
negating the safety aspect of the low voltage.
This may be the nub of your problem. There may be a fault in your
pondlights that a proper transformer would isolate but these
electronic devices do not. So the RCCD "sees" the fault through this
thing and trips.
Colin Stamp
The shape suggests it probably has a toroidal transformer in it. They
can have huge switch-on surges that trip MCBs of a much higher rating
than you would expect. You can get MCBs that are designed to cope with
bigger surges, but fiddling with the CU just to accommodate one awkward
transformer is a bit silly really. I generally assume that big toroids
aren't really compatible with MCBs.
See if you can find either a laminated or electronic transformer.
Cheers,
Colin.
Andy Dingley
> However as soon as I turn it on the MCB/RCD trips on my CU
Big inductive load. Try a C curve MCB, or even a different B curve MCB
(some are just twitchy after years of service).
Tabby
> On 28/10/10 17:56, JimM wrote:
>
>
>
> > Hi guys
>
> > Further to a previous post I've purchased the transformer in the link
> > below for my garden lights,
>
>
> > However as soon as I turn it on the MCB/RCD trips on my CU :( Originally
> > I thought the unit was faulty so I got a replacement sent but both units
> > have the same problem.
>
> > I'm running the transformer on a 30A ring main but it is through a 13A
> > plug is this likely to be the cause of the problem? If not any
> > suggestions of basics tests I could do to see if it the transformer
> > itself that is at fault (obviously the unit is sealed so I can't poke
> > around inside)
>
> The shape suggests it probably has a toroidal transformer in it. They
> can have huge switch-on surges that trip MCBs of a much higher rating
> than you would expect. You can get MCBs that are designed to cope with
> bigger surges, but fiddling with the CU just to accommodate one awkward
> transformer is a bit silly really. I generally assume that big toroids
> aren't really compatible with MCBs.
>
> See if you can find either a laminated or electronic transformer.
>
> Cheers,
>
> Colin.
Yes, its a 30A mcb though. Normally such things are used on 5 & 6A
circuits.
Possibles:
faulty unit from same batch - most likely
socket wired wrong, n/e confused
NT
The Natural Philosopher
surge. Kinder to the bulbs as well. Think yu can buy those separately.
John Rumm
> Hi guys
>
> Further to a previous post I've purchased the transformer in the link
> below for my garden lights,
>
> http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=380090544021&ssPageName=STRK:MEWNX:IT
>
>
> However as soon as I turn it on the MCB/RCD trips on my CU :( Originally
Well which, the MCB or the RCD or both (or have you got a RCBO)?
> I thought the unit was faulty so I got a replacement sent but both units
> have the same problem.
>
> I'm running the transformer on a 30A ring main but it is through a 13A
30A would suggest a fuse, rather than a 32A MCB...
> plug is this likely to be the cause of the problem? If not any
not a chance....
> suggestions of basics tests I could do to see if it the transformer
> itself that is at fault (obviously the unit is sealed so I can't poke
> around inside)
bit more info needed.
--
Cheers,
John.
/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/
Colin Stamp
>
> Yes, its a 30A mcb though. Normally such things are used on 5& 6A
> circuits.
Never underestimate the capacity of a toroid to trip an MCB. In the
worst-case, a cheap one will have virtually no inductance whatsoever,
just a few ohms of copper straight between L and N. They trigger the
magnetic (short circuit) bit of the MCB rather than the current overload
bit, so the actual MCB rating is less important.
>
> Possibles:
> faulty unit from same batch - most likely
Could be but, if it *is* a toroid, then I still wouldn't be surprised if
they don't "all do that sir".
> socket wired wrong, n/e confused
Surely the OP would have noticed that anything plugged into that socket
would trip the RCD?
Cheers,
Colin.
Tabby
> On 28/10/10 21:00, Tabby wrote:
>
>
>
> > Yes, its a 30A mcb though. Normally such things are used on 5& 6A
> > circuits.
>
> Never underestimate the capacity of a toroid to trip an MCB. In the
> worst-case, a cheap one will have virtually no inductance whatsoever,
> just a few ohms of copper straight between L and N. They trigger the
> magnetic (short circuit) bit of the MCB rather than the current overload
> bit, so the actual MCB rating is less important.
They need to be suitable for purpose, ie run ok on a 6A mcb.
Even badly behaved toroidals have ohmic primary resistance. A 100w
transformer with 10% regulation will surge upto 10x rated current when
inductance is temporarily zero. If this is the problem, the OP would
notice that sometimes it trips, sometimes it doesnt.
At the end of the day, if it wont work on a standard 30A breaker, its
little use to anyone, and either faulty or unfit for purpose. I'd
reject the goods as faulty, and refuse yet another one from the same
batch.
> > Possibles:
> > faulty unit from same batch - most likely
>
> Could be but, if it *is* a toroid, then I still wouldn't be surprised if
> they don't "all do that sir".
>
> > socket wired wrong, n/e confused
>
> Surely the OP would have noticed that anything plugged into that socket
> would trip the RCD?
>
> Cheers,
>
> Colin.
Yes - it just might be a forgotten socket that hasnt been used in
years.
NT
Dave Plowman (News)
> Further to a previous post I've purchased the transformer in the link
> below for my garden lights,
> http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=380090544021&ssPageName=STRK:MEWNX:IT
> However as soon as I turn it on the MCB/RCD trips on my CU :(
> Originally I thought the unit was faulty so I got a replacement sent
> but both units have the same problem.
By its shape, it's a real transformer. The inrush current on these can
trip an MCB as it can be very high for an instant. Do the instructions
cover this - like say stating a type C MCB is needed?
It shouldn't trip an RCD - that would suggest it's leaking to ground on
one leg. So faulty.
--
*Why do overlook and oversee mean opposite things? *
Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.
The Natural Philosopher
> In article <4cc9aac0$0$32102$ed36...@nr5-q3a.newsreader.com>,
> JimM <j...@home.com> wrote:
>> Hi guys
>
>> Further to a previous post I've purchased the transformer in the link
>> below for my garden lights,
>
>> http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=380090544021&ssPageName=STRK:MEWNX:IT
>
>> However as soon as I turn it on the MCB/RCD trips on my CU :(
>> Originally I thought the unit was faulty so I got a replacement sent
>> but both units have the same problem.
>
> By its shape, it's a real transformer. The inrush current on these can
> trip an MCB as it can be very high for an instant. Do the instructions
> cover this - like say stating a type C MCB is needed?
>
> It shouldn't trip an RCD - that would suggest it's leaking to ground on
> one leg. So faulty.
>
If its tripping an RCD its a sign of some high residual earth currents
(RCD on the edge anyway) plus a rather high inrush current.
Dave Plowman (News)
The Natural Philosopher <t...@invalid.invalid> wrote:
> > It shouldn't trip an RCD - that would suggest it's leaking to ground on
> > one leg. So faulty.
> >
> Mine dont have a ground.
> If its tripping an RCD its a sign of some high residual earth currents
> (RCD on the edge anyway) plus a rather high inrush current.
Can you explain how an RCD can trip on something like this with no ground
connection?
Ie, how can you get an imbalance on the current flow between line and
neutral?
--
*On the other hand, you have different fingers*
The Natural Philosopher
> In article <iae4np$ue7$3...@news.albasani.net>,
> The Natural Philosopher <t...@invalid.invalid> wrote:
>>> It shouldn't trip an RCD - that would suggest it's leaking to ground on
>>> one leg. So faulty.
>>>
>> Mine dont have a ground.
>
>> If its tripping an RCD its a sign of some high residual earth currents
>> (RCD on the edge anyway) plus a rather high inrush current.
>
> Can you explain how an RCD can trip on something like this with no ground
> connection?
> Ie, how can you get an imbalance on the current flow between line and
> neutral?
>
building or a high enough impedance in the live such that switch on
causes a mometary voltage drop in the live that is enough to trigger,
via all the capacitors in all the RFI devices between ground and live,
the RCD.
John Rumm
> In article<iae4np$ue7$3...@news.albasani.net>,
> The Natural Philosopher<t...@invalid.invalid> wrote:
>>> It shouldn't trip an RCD - that would suggest it's leaking to ground on
>>> one leg. So faulty.
>>>
>> Mine dont have a ground.
>
>> If its tripping an RCD its a sign of some high residual earth currents
>> (RCD on the edge anyway) plus a rather high inrush current.
>
> Can you explain how an RCD can trip on something like this with no ground
> connection?
> Ie, how can you get an imbalance on the current flow between line and
> neutral?
Its a known problem with most RCDs when they are "pre-sensitised" (i.e.
the nominal circuit leakage has used most of their trip current budget,
so they are very close to tripping).
The inrush surge can generate enough distortion of the supply waveform
to inject lots of harmonic noise into the supply. This is coupled to the
system earth via all the mains input filter in your appliances, and that
can briefly give you enough extra leakage. Note some older RCDs are
sensitive to the surge in their own right as well - even without the
help of external appliances. The same trip scenario can occur from
externally generated spikes etc as well.
Andy Wade
> By its shape, it's a real transformer. The inrush current on these can
> trip an MCB as it can be very high for an instant. Do the instructions
> cover this - like say stating a type C MCB is needed?
I wouldn't expect a single 100 VA transformer to trip a 32 A Type B MCB.
The instantaneous inrush current cannot possibly exceed the peak mains
voltage (about 340 V) divided by the resistance of the primary winding.
A 100 VA transformer will have a primary winding resistance of around
10 ohms (give or take a bit), so the worst-case current spike is about
the same as the continuous current-carrying ability of the MCB. To trip
the B32 MCB on the magnetic part of its characteristic needs a surge
current of at least 3*In, i.e. 96 amps. A transformer that could do
that would have to have a primary resistance of less than 3.5 ohms,
which takes you into the territory of transformers much larger than this
one. With a 5 or 6 A MCB it might be a different matter.
If an MCB-trip is occurring the transformer must be faulty - primary
shorted turns - or incorrectly rated - 110 V primary? - or connected
with primary and secondary transposed!
> It shouldn't trip an RCD - that would suggest it's leaking to ground on
> one leg. So faulty.
Or incorrectly connected - primary between L & E - stranger things have
happened...
--
Andy
Mike Tomlinson
>Further to a previous post I've purchased the transformer in the link below
>for my garden lights,
>
>http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=380090544021&ssPageName=ST
H'm. Likes his exclamation marks, doesn't he?
>However as soon as I turn it on the MCB/RCD trips on my CU :(
Is that with it submerged or out of the water?
You are fitting the mains plug to the correct wire, yes? :o)
--
Mike Tomlinson
harry
> In article <iae4np$ue...@news.albasani.net>,
> The Natural Philosopher <t...@invalid.invalid> wrote:
>
> > > It shouldn't trip an RCD - that would suggest it's leaking to ground on
> > > one leg. So faulty.
>
> > Mine dont have a ground.
> > If its tripping an RCD its a sign of some high residual earth currents
> > (RCD on the edge anyway) plus a rather high inrush current.
>
> Can you explain how an RCD can trip on something like this with no ground
> connection?
> Ie, how can you get an imbalance on the current flow between line and
> neutral?
>
> --
> *On the other hand, you have different fingers*
>
> To e-mail, change noise into sound.
In days of yore when earth leakage devices first came out, this was
major problem, now rectified. It was to do with transients running
about the system caused by stuff like fluorescent lights and capacitor
start electric motors.
Colin Stamp
> They need to be suitable for purpose, ie run ok on a 6A mcb.
Absolutely. What it needs to be and what it is are two different things
though :o)
>
> Even badly behaved toroidals have ohmic primary resistance. A 100w
> transformer with 10% regulation will surge upto 10x rated current when
> inductance is temporarily zero. If this is the problem, the OP would
> notice that sometimes it trips, sometimes it doesnt.
I don't think that quite follows. 10X rated current might be what you'd
get if you shorted the secondary, but that's not the same thing as peak
mains voltage divided by the primary DC resistance. Having said that, I
admit it is still probably a bit of a push for a 100VA transformer to
trip a 30A MCB.
>
> At the end of the day, if it wont work on a standard 30A breaker, its
> little use to anyone, and either faulty or unfit for purpose. I'd
> reject the goods as faulty, and refuse yet another one from the same
> batch.
Yep. Regardless of what it's tripping and why it's tripping it, getting
a new one of a different type is probably the only way out.
Cheers,
Colin.
Andy Wade
> On 29/10/10 02:21, Tabby wrote:
>>
>> Even badly behaved toroidals have ohmic primary resistance. A 100w
>> transformer with 10% regulation will surge upto 10x rated current when
>> inductance is temporarily zero. If this is the problem, the OP would
>> notice that sometimes it trips, sometimes it doesnt.
>
> I don't think that quite follows. 10X rated current might be what you'd
> get if you shorted the secondary, but that's not the same thing as peak
> mains voltage divided by the primary DC resistance.
You're quite right - that doesn't follow. If the transformer prim'y and
sec'y have roughly equal copper areas then half of the impedance will be
attributable to each winding. So in Tabby's example the worst-case
surge will be 20x rated (RMS) current, times 1.4 to allow for
instantaneous peak values.
> Having said that, I admit it is still probably a bit of a push for a
> 100VA transformer to trip a 30A MCB.
As I tried to say earlier that simply won't happen, unless there's
something wrong with it, or unless you've connected the secondary to the
mains (which does tend to make things go with a bang (DAMHIKT)).
--
Andy
Colin Stamp
Could be. I've come across plenty of instances of toroidals taking out
MCBs though. Probably smaller MCBs, though still pretty impressive.
My own main experience with them was long long ago, and the transformers
were bigger - 750VA auto-transformers. When I started testing one, I
just put a 13A plug on it and plugged into a the socket ring in the
company "lab". After a couple of switch-ons, the lab MCB on that ring
didn't just trip, it broke! It wouldn't stay latched on after that and I
had to replace it. The upstream building breaker feeding the whole lab
CU also tripped. I had to resume testing in the distribution panel
cupboard so I could keep resetting the breaker. Of course this was a
rush job and I had to find a way of making them MCB-friendly...
I can't remember the ratings of the MCBs, but I remember boggling at the
time. The DC resistance between the 0 and 240V taps was around 1 Ohm IIRC.
Cheers,
Colin.
Andy Wade
> Could be. I've come across plenty of instances of toroidals taking out
> MCBs though. Probably smaller MCBs, though still pretty impressive.
>
> My own main experience with them was long long ago, and the transformers
> were bigger - 750VA auto-transformers. When I started testing one, I
> just put a 13A plug on it and plugged into a the socket ring in the
> company "lab". After a couple of switch-ons, the lab MCB on that ring
> didn't just trip, it broke! It wouldn't stay latched on after that and I
> had to replace it. The upstream building breaker feeding the whole lab
> CU also tripped. I had to resume testing in the distribution panel
> cupboard so I could keep resetting the breaker. Of course this was a
> rush job and I had to find a way of making them MCB-friendly...
>
> I can't remember the ratings of the MCBs, but I remember boggling at the
> time. The DC resistance between the 0 and 240V taps was around 1 Ohm IIRC.
Yes, that all rings true and I've had similar experiences with large
transformers. The answer, for transformers built into equipment, is a
soft-start circuit with a beefy series wire-wound resistor, shorted out
by a relay or contactor after a suitable time delay - or a series NTC
thermistor if the transformer's going to see a continuous load. For
transformers in fixed installations the problem can usually be averted
by suitable choice of the overcurrent protection - Type C or D MCBs or
appropriate fuses, such as BS 88 gM motor fuses.
--
Andy
John Rumm
> On 30/10/2010 11:27, Colin Stamp wrote:
>
>> Could be. I've come across plenty of instances of toroidals taking out
>> MCBs though. Probably smaller MCBs, though still pretty impressive.
>>
>> My own main experience with them was long long ago, and the transformers
>> were bigger - 750VA auto-transformers. When I started testing one, I
>> just put a 13A plug on it and plugged into a the socket ring in the
>> company "lab". After a couple of switch-ons, the lab MCB on that ring
>> didn't just trip, it broke! It wouldn't stay latched on after that and I
>> had to replace it. The upstream building breaker feeding the whole lab
>> CU also tripped. I had to resume testing in the distribution panel
>> cupboard so I could keep resetting the breaker. Of course this was a
>> rush job and I had to find a way of making them MCB-friendly...
>>
>> I can't remember the ratings of the MCBs, but I remember boggling at the
>> time. The DC resistance between the 0 and 240V taps was around 1 Ohm
>> IIRC.
>
> Yes, that all rings true and I've had similar experiences with large
I find my 3kVA yellow tool transformer will take out a 20A MCB about 30%
of the time on switch on. Never done it on a 32A one though. (that's not
a torride though!)
> transformers. The answer, for transformers built into equipment, is a
> soft-start circuit with a beefy series wire-wound resistor, shorted out
> by a relay or contactor after a suitable time delay - or a series NTC
I did some work on a medium sized (10kW) broadcast amp once that used
this trick on its HT transformer[1]. The switch on sequence in that was
done in software - primary "step-start" contactor was closed first, and
then 200ms later the "start" contactor was closed to short out the
series resistance (and prevent it going puff!).
[1] This had mains on the primary and stepped that up to several kv on
the output. It was about 3' tall and had wheels on it. I seem to recall
it took a ramp and two people to get it into the case! In fact, IIRC
there is a piccie here:
http://www.internode.co.uk/temp/241.jpg
The transformer being the brown bit, bottom right. To give you some
scale the whole cabinet is about 6'6" at the tallest bit.
> thermistor if the transformer's going to see a continuous load. For
> transformers in fixed installations the problem can usually be averted
> by suitable choice of the overcurrent protection - Type C or D MCBs or
> appropriate fuses, such as BS 88 gM motor fuses.
--
Colin Stamp
> Yes, that all rings true and I've had similar experiences with large
> transformers. The answer, for transformers built into equipment, is a
> soft-start circuit with a beefy series wire-wound resistor, shorted out
> by a relay or contactor after a suitable time delay - or a series NTC
> thermistor if the transformer's going to see a continuous load. For
> transformers in fixed installations the problem can usually be averted
> by suitable choice of the overcurrent protection - Type C or D MCBs or
> appropriate fuses, such as BS 88 gM motor fuses.
Yep. Unfortunately, the only control we had over the over-current
protection was to put a warning in the customer documentation, so we
knew that would come to nothing. I tried various sizes of thermistor
first, but they took too long to cool down, so the breaker still tripped
if the transformer was switched off for a few seconds. I ended up with
the big timed resistor option but it was too late to cook up a suitable
timer and we had to buy pay-through-the-nose timing relays. Not too
elegant but at least none of the support calls had anything to do with
tripped MCBs.
Cheers,
Colin.
Tabby
> On 29/10/2010 16:49, Colin Stamp wrote:
>
> > On 29/10/10 02:21, Tabby wrote:
>
> >> Even badly behaved toroidals have ohmic primary resistance. A 100w
> >> transformer with 10% regulation will surge upto 10x rated current when
> >> inductance is temporarily zero. If this is the problem, the OP would
> >> notice that sometimes it trips, sometimes it doesnt.
>
> > I don't think that quite follows. 10X rated current might be what you'd
> > get if you shorted the secondary, but that's not the same thing as peak
> > mains voltage divided by the primary DC resistance.
>
> You're quite right - that doesn't follow. If the transformer prim'y and
> sec'y have roughly equal copper areas then half of the impedance will be
> attributable to each winding. So in Tabby's example the worst-case
> surge will be 20x rated (RMS) current,
right - though I'm NT
> times 1.4 to allow for
> instantaneous peak values.
... which is still 10x normal current twice a cycle.
> > Having said that, I admit it is still probably a bit of a push for a
> > 100VA transformer to trip a 30A MCB.
>
> As I tried to say earlier that simply won't happen, unless there's
> something wrong with it, or unless you've connected the secondary to the
> mains (which does tend to make things go with a bang (DAMHIKT)).
Ha. You should be able to run a 200w transformer on a 2.5A breaker for
ever more.
NT
Andy Burns
> I'm NT
Eh? You changed to "Tabby" months ago ...
Tabby
/I/ didnt. I just cant change it back for some reason.
NT
Ron Lowe
What?
Your posting name is surely under your direct control, isn't it?
It's configured on your news client.
Did you just wake up one morning, stub your toe and suddenly you were
'Tabby'?
--
R
Andy Burns
> Your posting name is surely under your direct control, isn't it?
Apparently not if you post from Gurgle Gropes, I know that gmail allows
you to create various aliases, presumably it's picked up a temporary one
and is using it permanently?
Tabby
> Ron Lowe wrote:
> > Your posting name is surely under your direct control, isn't it?
I don't use a news client
> Apparently not if you post from Gurgle Gropes, I know that gmail allows
> you to create various aliases, presumably it's picked up a temporary one
> and is using it permanently?
Seems so. I looked at all the news access settings when it happened,
but doesnt seem I can change it. I dont use gmail. Ah well, not the
end of the world.
NT
Andrew Gabriel
Andy Wade <spamb...@maxwell.myzen.co.uk> writes:
> On 29/10/2010 16:49, Colin Stamp wrote:
>> On 29/10/10 02:21, Tabby wrote:
>>>
>>> Even badly behaved toroidals have ohmic primary resistance. A 100w
>>> transformer with 10% regulation will surge upto 10x rated current when
>>> inductance is temporarily zero. If this is the problem, the OP would
>>> notice that sometimes it trips, sometimes it doesnt.
>>
>> I don't think that quite follows. 10X rated current might be what you'd
>> get if you shorted the secondary, but that's not the same thing as peak
>> mains voltage divided by the primary DC resistance.
>
> You're quite right - that doesn't follow. If the transformer prim'y and
> sec'y have roughly equal copper areas then half of the impedance will be
> attributable to each winding. So in Tabby's example the worst-case
> surge will be 20x rated (RMS) current, times 1.4 to allow for
> instantaneous peak values.
I think this is only accounting for half the potential toroidal
inrush current.
The problem is that the steel used has very high hysteresis (probably
an unwanted side effect of finding steel strip with suitable properties
to wind into a toroid shape), and that there's not even a micro air-gap
in the core. The high hysteresis means that you'll most often have
high residual mangetism left in the core from last switch-off (assuming
it was sudden and not dimmed down). When you apply voltage, and current
starts to flow, the core now very likely saturates which would result in
the current flow you mention above. However, the residual field also
collapses generating an EMF from the transformer, which can add to the
mains voltage. If it collapses at the same rate of change as normal
field changes, I think you have a worse case of almost 240V * 2 across
the primary resistance. However, I think why the rate of collapse is
related to the normal rate of change of field in normal operation, so
it might be even worse if the collapse is faster.
30 years ago, I fully understood this, having had similar problems
with a toroidal transformer in a project I was building, and I went to
ask one of our physics professors. However, I really can't recall all
the details now. The other thing is that it's not a quarter or half
cycle surge, but can take several cycles to get the hysteresis curve
in the steel core back centred on the graph axis cross-hairs, so
the primary current draw can be high for some cycles, but dropping
back to expected levels.
I have one set of 10 x 10W halogens on a 100W toroidal transformer
on my lighting circuit. I built that with an NTC thermistor to limit
the inrush current, and it never tripped the 6A Type 2 MCB I used to
have (nor the Type C I now have). I didn't try it without the
thermistor. If you do this, bare in mind the thermistor runs hot,
and they can explode if there's a fault. Mine is in a metal box for
these reasons.
Other things that might help in theory (but I haven't tried):
Use a dimmer with soft start (or better still, both soft start and
soft off), although you don't want to run it dimmed except during on
and off cycles. Dimmer might not survive driving the transformer though.
Degauss the core before switchon. Leaking a tiny AC current through
the primary whilst switched off would do this, but safety concerns about
having a circuit which is supposed to be switched off still slightly live
probably rules this out.
Still thinking on this one, not 100% sure if it will help, but switching
the transformer off when it has no load may make this worse. So if you
can get it to come on once without tripping, and you ensure it always
has a load connected, you might find the problem goes away, but this is
really guessing. Would be interested to know if you can prove/disprove
it though.
Note that a zero crossing switch will not help.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]