Rcds and transformers

[m4tty]

Hi,

Simple question for you guys in the know.

If an rcd is looking for an imbalance between live and neutral, what would happen if the imbalance was the other side of a transformer. Would the imbalance be seen on the primary side which occured on the secondary side?

Sorry if I'm being thick but would like to know what happens.

Thanks

 

[Andy™]

imbalance on secondary of transformer would be no imbalance on primary

 

[Sidewinder]

Immediate thought that popped into my head today...

Not a clue!

Bad day! bad day explode

I think it may depend on the exact configuration of the transformer and the load seen by the supply.

Though my head is in the shed at the moment and, I'm, still,... van less!

 

[m4tty]

WHat about a short circuit between live and neutral on the secondary. Would that be seen on the primary? Thanks

 

[Doc Hudson]

Assuming we are talking about standard 12v transformers used in domestic lighting, or 1:1 isolating transformers used in shaver sockets. There is no path back via earth to the star point of the supply generator as the earth is isolated at the transformer. So no route for any current to flow from either live or neutral. So an imbalance on the secondary cannot occur. Current has to flow somewhere to create an imbalance.

Doc H.

 

[m4tty]

Van still not sorted sidey?

 

[Sidewinder]

m4tty,

I think that it will all depend on the transformer design.

IF there is a pri-sec link such as a common N then it could be seen.

Thing is there are so many different configurations then there is not just one answer.

Having thought I think that Andy has the correct answer for the type of transformer he has in mind.

If it is a true isolating transformer then no secondary faults should show on the primary and the converse.

---------- Post Auto-Merged at 18:19 ---------- Previous post was made at 18:18 ----------

Nope turbo was due in today and did not arrive garage were ringing me to tell me when I was ringing to chase them!

Sir KME Knight of the Realm (well forum) and all round good egg of the forum is helping me out tomorrow by taking me to the job and he is gonna give me a hand then I am helping him tomorrow aft!

 

[kme]

"Sir KME"???????

That has a certain ring to it..........Does it have an increased pay scale attached???? :tongue in cheek

 

[Sidewinder]

:slap ROTFWL :red card :x

 

[kme]

 

[Evans Electric]

With a CTE Transformer would, I presume , trip an MCB on the primary and an earth fault would have to trip an RCD , again I presume.

And as said , an isolating Transformer is what it says on the tin , no earth and only an induced connection with the mains.

Asks some good ones doesn't he ?

 

[Sidewinder]

He does mate he does!

Good though it gets us all thinking!

 

[Andy™]

With a CTE Transformer would, I presume , trip an MCB on the primary and an earth fault would have to trip an RCD , again I presume.And as said , an isolating Transformer is what it says on the tin , no earth and only an induced connection with the mains.

Asks some good ones doesn't he ?

no transformer will not trip the RCD on primary if there is a fault on secondary. unless the transformer is faulty (the neutral of primary must be shorted to secondary), and there is a fault that will then use the neutral through RCD as a fault path

if your still not sure, go draw a diagram of both CTE & isolating, and follow path through RCD to coil then back to RCD - unless power from secondary can get into wiring between RCD & primary coil, then there is nothing for RCD to detect

 

[Sidewinder]

That makes sense Andy as long as there is no N link in place which some can have.

For Isolating then definite.

CTE would have to think/draw, but initial thought is that you are totally correct.

Can't do thinking tonight.

Totally FUBAR tonight.

There are other types of transformer though that may not exhibit this behaviour.

 

[ProDave]

WHat about a short circuit between live and neutral on the secondary. Would that be seen on the primary? Thanks

That would be seen on the primary as over current and should trip the MCB. That will still not trip an RCD.