Understanding breakers and RCD, un-vented cylinders

[jones82]

Hi

I'm a Plumber and heating engineer and a new member to your forum.

I have a basic knowledge of electrics but I'm curious about how breakers and Rcd's work and whether they prevent fires.

My basic understanding is the breaker trips if more power is been drawn than is safe and an RCD senses an earth fault and trips almost instantly.

So on a modern installation, with breakers and RCD would an electrical fault which could have led to a fire on an old installation be prevented?

Recently I attended a faulty economy 7 immersion on an unvented cylinder. The live had completely come away from the element, either by burning out or someone interfering with it, surprisingly the electrics hadn't tripped and there was signs of scorching inside the immersion cover.

This also leads onto another question. I'm G3 qualified and regularly work on and install un-vented cylinders but I've had very little experience with direct economy 7 models. I usually install and repair in-direct units.

It is my understanding that an un-vented immersion is different from a standard one and I always contact the manufacturer to get parts specific to the model I am working on. However if someone replaced an un vented immersion stat with a conventional one, what problems could this cause?

Thanks for your time

 

[PEG]

Hi dude,not sure what the difference is between short-circuit,and over current

As for the immersion and thermostat,there have been various incremental changes,regarding new fit spec,some regulatory,some driven by manufacturers instructions and best practice,such as fixed or non-adjustable thermostats,and reset/non-reset trips.

The latter providing a quandary,on systems where immersions fitted as a summer back-up,on tanks/stores with supplementary heating inputs,which take the temperature above the trip.

 

[I2C]

not sure what the difference is between short-circuit,and over current

Short Circuit
In simple terms, a short-circuit is where the line & neutral conductors touch each-other, resulting in a huge amount of current flowing through the circuit (and circuit-breaker) which causes it to rapidly trip.

Over-Current
In simple terms, an over current occurs when the rating of the circuit-breaker is slightly exceeded, but not massively exceeded. For example, if a ring-final circuit (fed by a 32A circuit-breaker) had several appliances plugged-in drawing a total of 33A, the circuit breaker will not instantly trip despite the current-draw exceeding the protective-device's current, referred to as an over-current. The circuit breaker can eventually trip because of the bi-metallic strip used for this purpose in the circuit-breaker, but this will take a substantial amount of time compared to a short-circuit.[/QUOTE]

 

[PEG]

Hi friend,i was not having a pop,but you are using one electrical nomenclature phrase,and one slang or anachronistic phrase,to describe one occurrence.

The dictionary definition of "short-circuit",is a deviation of current,of low resistance,which is not necessarily to neutral,and not descriptive of impedance or path.

I respect that you prefaced the post,regarding "basic terms",hence my smiley.

I am not short of someone to explain these terms,however simple the terms are.

Respect and a Happy Christmas.

 

[I2C]

Hi friend,i was not having a pop,but you are using one electrical nomenclature phrase,and one slang or anachronistic phrase,to describe one occurrence.

The dictionary definition of "short-circuit",is a deviation of current,of low resistance,which is not necessarily to neutral,and not descriptive of impedance or path.

I respect that you prefaced the post,regarding "basic terms",hence my smiley.

I am not short of someone to explain these terms,however simple the terms are.

Respect and a Happy Christmas.

As you've mentioned, I'm drastically simplifying my terminology in order to provide better explanations subjectively. I've stated previously that I'm fully-aware of the correct terminology.
If you'd like to be specific, you stated "current of low-resistance" which is incorrect; the correct terminology would be "A short-circuit between line & neutral of negligible-impedance".

Also, you're correct in stating that short-circuits are "not necessarily to neutral" (such as three-phase) however this is out-of-context considering the original post and I'd like to prevent confusion where possible, hence my simplistic terminology and explanations.

Regardless, merry Christmas

 

[PEG]

Hi, i started with a specific "dictionary definition",that is Oxford variety,and not Google and it says,what i said.

I have always had a problem with the term "short circuit",and this is the only reason i am whining on

I think it stems from me nan,who as a kid,would describe absolutely any deviation from correct operation,in machinery or person,as a "short circuit" ....that,and general grumptitude

 

[ipf]

Hi, i started with a specific "dictionary definition",that is Oxford variety,and not Google and it says,what i said.

I have always had a problem with the term "short circuit",and this is the only reason i am whining on

I think it stems from me nan,who as a kid,would describe absolutely any deviation from correct operation,in machinery or person,as a "short circuit" ....that,and general grumptitude

.....and I always thought a 'short circuit' was less than 5m in length....you live and learn, eh!

 

[I2C]

Hi, i started with a specific "dictionary definition",that is Oxford variety,and not Google and it says,what i said.

I have always had a problem with the term "short circuit",and this is the only reason i am whining on

I think it stems from me nan,who as a kid,would describe absolutely any deviation from correct operation,in machinery or person,as a "short circuit" ....that,and general grumptitude

This definition is taken directly from the Wiring Regulations (BS7671 17th Edition Amendment 3). I'm not sure what an "Oxford Dictionary" states, but regardless the IET publications are the proper reference.

However I do understand, many people use the term 'short-circuit' incorrectly / naively to describe faults. Everyone has done at some point (including myself).

I once worked with an electrician once who told me that using a AVI (Approved Voltage Indicator to GS38) between line-neutral would create a short-circuit! With this said, I promptly explained that the AVI that I was using had an internal resistance of 200kOhms which results in a perfectly safe and easily calculated current-flow of around 1mA. And yes, I'm aware that I'm sad enough to read the datasheets.

In fact, when I first began training as an electrician I honestly believed that transformers, electromagnets, and electric-motors where a short-circuit due to their measured low resistance! I seriously couldn't understand why circuit-breakers didn't trip when a motor was switched on. Of course, this was before I understood inductive-reactance, and therefore impedance.

My advice - nobody knows everything (including myself) and mistakes result in improved knowledge. I've made comments here which where naive and quickly corrected by other members, for which I have remained humble and learned from them. Don't be put off, no question or comment is silly because we all can only improve. That's why I love this forum, because everyone is so understanding and respectful, with all members having an incredible combined-knowledge.

 

[davesparks]

.....and I always thought a 'short circuit' was less than 5m in length....you live and learn, eh!

A short circuit is one where the normal circuit load is bypassed by a low resistance path where current can flow from one phase to neutral or to another phase.
Anything which results in the flow of current to earth is a fault

 

[I2C]

A short circuit is one where the normal circuit load is bypassed by a low resistance path where current can flow from one phase to neutral or to another phase.
Anything which results in the flow of current to earth is a fault

With all due respect, your definition of a 'short-circuit' is almost correct, without using correct terminology. I've given the indisputable definition directly from the Wiring Regulations (BS7671 Amendment 3) regarding short-circuits previously for reference.
You're correct in mentioning that a short-circuit is between live-conductors (live conductors meaning the conductors which carry current under normal conditions), between line-neutral / line-line with 3-phase systems, however it's important to consider the total opposition to current in an AC circuit (impedance), rather than simply the resistance.

Also, earth-faults are distinctly different from short-circuits. By their very nature, they require additional considerations (such as the requirement for 'additional protection', with the voltage represented as U0, and varying depending on the earthing-system) amongst other factors.

 

[electric al]

Looking at the picture of the stat I would say there should be a reset button underneath that sticker stating ' factory seal '

 

[ipf]

A short circuit is one where the normal circuit load is bypassed by a low resistance path where current can flow from one phase to neutral or to another phase.
Anything which results in the flow of current to earth is a fault

Thanks for the quote and info, dave. You really do live and learn

 

[camerabloke]

.....and I always thought a 'short circuit' was less than 5m in length....you live and learn, eh!

I thought short circuit was something to do with Jonny 5