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

 

[mattg4321]

As far as I know there is no difference between an unvented immersion/stat and a conventional one.

An MCB or RCD normally won't trip if theres a loose connection. The connection will heat up and eventually burn out.

 

[Wilko]

Hi Jones and Welcome to the Forum!
I'm sure you'll get plenty of help here, and there's several interesting questions you've raised. Re the L disconnected from the element - if it had touched E or N then one or both of the protective devices should've tripped. Any signs of overheating?

 

[jones82]

Thanks


Just out of interest, I kind of assume I'm safer when I'm working on a house with modern electrics, so new consumer unit, rcd etc

As if there's a fault then theoretically it will trip before it does any damage to either me or property etc, is this correct?

I've always thought that if there was to be dodgy electrics and the chase of a shock then it would be on some old bungalow with fuse wire etc instead of breakers and rcds

 

[jones82]

Hang on I'll try post a pic

 

[jones82]

This is the immersion I mentioned

 

[mattg4321]

Look like someones bodged that.

Where does the cable coming out of the left hand side of the stat go? This should be connected to the element - where the damaged cable is.

 

[jones82]

Ahh yes I see what you mean, its connecting onto one of the element connections. Perhaps someones tried to reconnect it or bodged it connecting a new stat?

To be honest I just took one look at it and said new immersion. I'm pricing one up now, Gledhill Sh010 £60 online.

 

[davesparks]

Thanks


Just out of interest, I kind of assume I'm safer when I'm working on a house with modern electrics, so new consumer unit, rcd etc

As if there's a fault then theoretically it will trip before it does any damage to either me or property etc, is this correct?

I've always thought that if there was to be dodgy electrics and the chase of a shock then it would be on some old bungalow with fuse wire etc instead of breakers and rcds

You are only safe when you isolate the supply correctly following the safe isolation procedure including locking off the supply and using a proper voltage indicator, not a multimeter, voltstick or neon screwdriver.

And RCD is designed to reduce the severity of an electric shock to a level which is unlikely to be fatal to a healthy adult. If you touch a live conductor which has 30mA RCD protection then you will receive an electric shock which could be quite painful but the RCD should detect this and disconnect the supply before it becomes fatal.

 

[Des 56]

Mr Heating man, have a look at this video,it is thought our wiring regs may introduce them soon

 

[Toneyz]

All ready for the big sales push if in 18th then

 

[UKMeterman]

You must use the manfacturer's stat when replaceing, it has at least two levels of protection. If all the protection electrical and mechanical fails the cyliner could explode and then you have 1/4 ton of steam at say 10 bar, this could demolish the house.

 

[PEG]

Hi bud,and welcome.

I would like to add,that the fact that a property has a shiny,new DB,with a line of RCD's...does NOT guarantee survival,on grasping a live part!

There is no certainty that those devices will operate correctly,or even in time,and the testing that sparks do,ONLY ascertains proper operation,at the time of testing.

There are a few other scenarios,where these devices will not help,such as if you received an electric shock from live,to neutral,as you would only then be sensed as a tiny load,but could be knacked just the same

Get yourself an approved voltage indicator,which not only is the tool for testing these areas properly,but,will immediately give you 20 extra respect points,should any spark slide past

 

[Darkwood]

Looking at the pic, this issue here is water ingress from the tank (possible steam leak when pressured), the screws, threaded bar have all corroded with moisture but the thread top that holds the lid is still shiny that would suggest the nut holding the lid on had not been exposed to the moisture ... you need to figure out why the internal parts are corroded and where the moisture is coming from, I would replace the element and stat and pressure test the tank before using .... as goes the electrical queries then you have had plenty of informative answers

 

[I2C]

I'll explain the following in basic terms for better understanding (other electricians, I know my terminology but please ignore for this post):

RCD (Residual Current Devices)
- Trips during earth-faults
- Doesn't trip during short-circuit faults
- Doesn't trip during over-current faults
An RCD measures how much electricity goes-out and how much comes-back (through the line & neutral). If there's a difference, it presumes electricity must be going to ground (through someone or something) and trips.

MCB (Miniature-Circuit-Breaker
- Trips during short-circuit faults
- Trips during over-current faults
- Doesn't trip during earth-faults
A MCB only measures how much electricity goes-out (through the line) to the circuit. If more electricity goes-out than the MCB expects, it trips.
The MCB has two ways of tripping - if the amount of electricity going-out is only slightly more than it expects, it's called a "over-current" which heats-up a bi-metallic-strip causing it to bend (like a thermostat) causing it to trip; if the amount of electricity going-out is way more than it expects, a solenoid operates causing it to trip.

RCBO (Residual-Current-Breaker-Operated)
- Trips during short-circuit faults
- Trips during over-current faults
- Trips during earth-faults
An RCBO combines an MCB with an RCD into one single device, providing all of the functions of both devices. These devices ensure that any type of fault will only trip one circuit instead of several, however they are usually much more expensive and therefore not commonly used.

Un-vented / Vented Immersion
I'm fairly sure that both types of cylinder use the same immersion and thermostat. Besides, it wouldn't make sense (economically) for a manufacturer to produce two types, rather than just one.

 

[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