Mem Eaton B-Type Mcb For 5Kva Transformer

[southsparks]

Came across a "tripping" problem today on a visit to a teaching workshop, a 5KVA "Blakley" transformer unit providing 110Volts to socket outlets for students, double-wound centre-tapped(as a site transformer,) fed from dis-board on 20 Amp circuit, with MEM/Eaton B-type RCBO. Am I correct in thinking it ought to be at least a C-Type, as current in-rush too much for B type? 

This happens every few switch-ons on Blakely unit. Really only want to replace MCB part, and re-use RCD bit. Is it also possible

to swop-over RCD to new MCB, without damaging it? This would be a field-fitted job.

Had considered using a D-type MCB, but a bit wary, as not sure if E-loop impedance low enough. The transformer is less than two metres from dis-board and wired in 4mm I think. Not sure how they managed to use this transformer for six years or so.

Quite sure B-type breakers quite unsuitable for this sort of equipment supply, but would be grateful if somebody could confirm that for me.

Thanks.

 

[Phoenix]

As a transformer it should be on a C type. D type is not normally necessary.

Seen a large school site that has slightly larger ones off a panel board. 8x SP 50A MCCBs feeding the units in 8 labs... old school engineering practice I presume.

You should note however that there is little point it being on an RCBO. As it will only monitor faults on the primary side. For those not in the know what goes on in the blakely unit is a double wound tx, which is centre tapped to earth... but through a delibrate resistance in the K-ohms range... Shock to earth is limited to 115v @ about 10ma if I recall correctly. Faults are disconnected by a earth leakage shunt trip set at 5mA or 1.25mA (for the later the conductor is looped through the sensing coil 4 times)

It is possible to swap the pods over on memshield 2, but I can't remember off hand how, I'm sure there's you tube videos though. I would personally just go to a MCB unless the cable is buried in wall, etc

In effect its a IT system, and the 3 neon testers will indicate no earth which worries the odd site maint. dept. Single pole switched socket outlets should not be used

 

[southsparks]

Thanks for all the advice. Today I was on site and replaced B type breaker for C type, but outcome just the same. Have taken test readings and reckon I could get away with a D type breaker, in fact the makers of the unit, Blakley, have suggested using a 32A D-type, assuming circuit cables adequate. I found the Zs earth-loop impedance at transformer to be 0.59 ohms, giving fault current of 405 Amps. Probably OK for a c-type 32A MCB, but not a D type, so may have to leave RCD pod on breaker. You can indeed swop over the RCD pod on site, quite easily. This would clear any lower current earth-faults quicker than just the MCB could.

Would any fault-voltages be transmitted to output circuits, as all earthing is joined together, in the event the input breaker slow-acting? Best to fit RCD if its not affected by current inrush I reckon, as it was fitted by installers, and I don't like the idea of removing any RCDs fitted by them.  You never know.

Have found the 230V output Blakleys do have integral RCD usually 5Ma, sometimes just 2.5Ma. Much too sensitive to use computers on! They need, as you say, DP sockets on them, as "neutral" is 115V to earth. It really confuses any test-instuments you may try and use, Blakley make a dedicated one just for their equipment.

Guess if you catch ac mains cycle near its minimum, it will reset without  protest. Got to be quick though, as ac voltage only takes 5Ms to reach max. from zero I believe!

Southsparks.

 

[Sidewinder]

Thanks for all the advice. Today I was on site and replaced B type breaker for C type, but outcome just the same. Have taken test readings and reckon I could get away with a D type breaker, in fact the makers of the unit, Blakley, have suggested using a 32A D-type, assuming circuit cables adequate. I found the Zs earth-loop impedance at transformer to be 0.59 ohms, giving fault current of 405 Amps. Probably OK for a c-type 32A MCB, but not a D type, so may have to leave RCD pod on breaker. You can indeed swop over the RCD pod on site, quite easily. This would clear any lower current earth-faults quicker than just the MCB could.

Would any fault-voltages be transmitted to output circuits, as all earthing is joined together, in the event the input breaker slow-acting? Best to fit RCD if its not affected by current inrush I reckon, as it was fitted by installers, and I don't like the idea of removing any RCDs fitted by them.  You never know.

Have found the 230V output Blakleys do have integral RCD usually 5Ma, sometimes just 2.5Ma. Much too sensitive to use computers on! They need, as you say, DP sockets on them, as "neutral" is 115V to earth. It really confuses any test-instuments you may try and use, Blakley make a dedicated one just for their equipment.

Guess if you catch ac mains cycle near its minimum, it will reset without  protest. Got to be quick though, as ac voltage only takes 5Ms to reach max. from zero I believe!

Southsparks.

I am doubtful that your figures are correct.

A 5 Mega what?

Do you mean 5mA, 2.5mA, & 5ms?

Remember NASA wrote off a space craft for a smaller error than the difference between a milli, m, & a Mega,M!

Such SI unit prefixes are very important especially m & M, as they are both relevant and used in electrical installation resistance measurements, so the distinction between the two must be preserved, as getting them wrong could kill someone.

 

[southsparks]

Yes thanks Gold I did mean milli-amps, but it was purely a reference to other equipment I have seen, and not any specification that I am requesting.

I got the upper & lower case settings on key-board mixed up. Rest assured nobody is going to be adversly affected!

The main problem I have at the  moment is to find the ohmic value of the primary winding of this 5 KVA transformer, I have got a test-reading of 0.3 Ohms,

which seems far too low, going by Ohms Law, it should be around say 15 Ohms, that would give you 5Kw. on a resistive load. 0.3 Ohms direct onto 230 volts

would give a current flow of nearly 700 Amps! I realise inductive loads work differently to resistive, and different calculations are needed to work out

impedance. Too long since I was at College.I am trying to get correct resistance values from Blakley, as they surely must know what it should be as they make

them. This a strange fault to me, and still trying to work out why the transformer sometimes can be turned on without any problem, then you get big bang from

MCB which sounds just like a very bad short-circuit. This is the first Blakley unit I have worked on where this has happened, and can't believe it is purely a

question of an incorrect selection of mains breaker type. I don't like giving up, as it might come to having to replace transformer, getting on for £1K I believe.

Looks like I will have to fit the  type D MCB as suggested by the makers, and if still goes bang, over to them. It has been in service six years so something must

have happened in last three weeks when this problem first flagged-up. They would like the unit fixed, so the trainees can use the 110V points.

Southsparks.

PS, sorry about typing, it wasn't like that on screen before I sent it. My computer seems to do that, or maybe router is faulty?

BS.

 

[southsparks]

Thanks again Gold.  Have heard from makers, and they quote 0.45 ohms as"impedance" of primary.Wanted to check if my reading correct.

I realise impedance is not the same as resistance. Both hold up current-flows, but in quite different ways. 

They recommend a 32A type D MCB for the supply so will at least try that.  Will refit RCD module as not sure if I can get Zs up to requirements.

Also that is the only RCD in system, but aware it would not detect any earth-faults on secondary. Hope new breaker does trick. Blakley engineer

agreed with me its all about catching AC cycle at max peak every so often, causing high current inrush on switch-on.

Have had similar problems in past with welders and even 12V lighting transformers to domestic lighting, the big wire-wound type.

Southsparks.

 

[Phoenix]

The fact that its started causing problems only recently does not have to indicate an equipment fault. There could have been changes to the DNO network or even the sites own distribution which has resulted in L-N impedance changing, this would effect inrush current.

 

[southsparks]

Thanks Phoenix. Never considered that possibility. It has occurred to me that the Blakley has simply been left switched on since it was

first energised. Maybe now its something to do with a new staff member being there and switching it off and on. Power-cuts? that would

entail a reset if breaker tripped when power restored. Have read that transformers can draw up to 40 times normal current on switch-on.

Don't think my clamp tester would be fast enough to register that current. Will be interested to see how much it draws under no-load

conditions. I'm afraid I've forgotten how to work-out current on AC circuits with impedance given as 0.45Ohms. DC easy, as just Ohms Law

I reckon. Come to think of it, we electricians frequently do an impedance test on the DNO's transformer, but that's for a supply, not a load.

Can't work out if it could be done in reverse. All other Blakleys I've worked on are switched via MEM 32/45 Amp knife-blade style isolators

backed-up by BS 88 HRC fuses. Best way I reckon. MCB's really not the best way, especially B-Type, I tend to regard as for domestic use.

Southsparks.

 

[southsparks]

Just an up-date to my post, fitted 32Amp type D MCB,(RCBO) but still the occasional tripping. Not so bad, and no loud bangs. Have suggested to users to leave it on

as it only uses a small amount un-loaded (0.6 Amps).Perhaps its the RCD doing it. By the way, having trouble finding this post on the Forum site. Where does it go?

Southsparks.

 

[roys]

Just click onto your name and click 'find content', there is a couple of ways to do it but for me that is the easiest way when you know you have posted on a topic.

 

[Dairyspark]

Working out current from impedance is the same as ohms law, just change Resistance ® for Impedance (Z) I=V/Z

 

[southsparks]

Thanks for further advice. Not heard of any more nuisance tripping for nine days now. Told them to leave it on all the time.

Was worried that  RCD may trip it out, but see no reason why it should, primary windings insulation test at 500 volts was over a giga ohms.

Had to leave RCD in as not sure earth PFC would be enough to trip type D MCB on its own. It would probably have taken several seconds to

do so, not "instantaneously" as Regs require, long enough to cause problems I would have thought. 

Not sure if I'm doing that calc. on impedance right, get 511 amps on Z=0.45 ohms @ 230V. Doesn't power factor come in to it somewhere?

Actual steady current draw with no load is actually 0.6 Amps. On the other hand, a Zs test on the  mains would give that much higher current, which

is what happens here, initially when you first switch it on.

Didn't think you could use Ohms Law on impedance. Thought it was a complicated calc. with "vectors" involved. So long since I was at Tech.

Southsparks.

 

[binky]

Thanks again Gold.

there's a lot of gold  members on this forum - ' oh yeah baby' :slap

 

[southsparks]

"There's old electricians, and there's bold electricians, but there's no old, bold, electricians."

Not sure how that describes me, just old & not too bold these days!

(That was originally applied to pilots, in some book I read years ago).

Maybe, but what about this, "she was just a farmers daughter, but all the horse manure".

Its how you say it if you can't get it.

SP.