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Myser

AskAsk said:

i don't know if you can see the thermostat wiring separately?

Did the electrician send you the photos?

Jeepers_Creepers

There are two separate switches for these elements?

To reiterate, from what has been said by your sparky this is not an issue of too much current being drawn or of a short-circuit in the wiring which would trip the immersions' circuit breaker/fuse (MCB), it's one of 'leakage' of a tiny current which is tripping the RCD in your CU (Consumer Unit).

Yes, it's ok for a person to replace like-for-like - as long as they do a competent job, of course. And you can assume they did or else your sparky would have picked up on this.

fenwick458

Mutton_Geoff said:

I thought anything with heavy current heating elements were inherently "leaky" to earth (eg immersions and ovens) and these should not be connected to the 30ma RCB protected side of the consumer unit. Could be the new ones are just more leaky than the old ones before they failed.
30ma protection is to save life if live terminals are touched. This can't usually happen on hard wired/spur type devices so they don't need the same protection.

Not true. Although I often hear a lot of "older" electricians telling me this, it's not an excuse for not connecting something to an RCD. are you suggesting it's ok to just have a "little bit" of earth leakage from the immersion element, onto the cylinder, and to various other copper pipes...... 
I don't see what the problem is here anyway, the OP has got the diagnosis from the electrician, the elements are both shorted to earth and are scrap. get back onto Dyno-rod and tell them the 2 elements they installed are faulty, simple. not sure why his diagnosis is not being believed, and now people are suggesting that they may be wired wrong. you'd think the electrician would know how to wire them wouldn't you and would have spotted if they were wired up wrong??
starting to make sense now why the original installer said you had to wait an hour till you could turn them on, maybe he bent them accidentally when fitting and had a feeling they were going to short out so wanted to be long gone before you realised

Mickey666

AskAsk said:

what does 'leakage' of tiny current means?

In a mains circuit, all the current that comes ‘down’ the live wire goes through the connected device and ‘returns’ through the neutral wire.  If the current returning along the neutral wire is less than the current coming down the live wire then it means it must be ‘leaking’ to somewhere else.  That leakage indicates some sort of fault because it shouldn’t happen.  Furthermore, if that current is leaking through a person touching a live wire then they will get an electric shock.
An RCD constantly monitors the current in the live and neutral conductors and if they are not equal, meaning a ‘leakage’ to somewhere else, then the RCD will trip and cut off the power.  RCDs typically trip at around 30mA of leakage.  This figure is chosen because it is non-fatal in almost all cases and the RCD trips so quickly that a shock would not be noticed and almost certainly not dangerous.  It’s a very powerful safety device.


(PS:  The above is a very basic description of the principle of current ‘leakage’ - hopefully the pedant experts will be gentle with me )

Myser

AskAsk said:

the electrician says that both elements are 'shorting to earth', do you know what this means?

It means that the elements have broken down internally and they are leaking current from the live/neutral to earth.  This is very unusual for new elements.  You usually associate this problem with aged/faulty elements.

Both elements causing the RCD to trip is rather suspicious and could be an installation error or the thermostats, if they need wiring in separately

Myser

AskAsk said:

thank you.  the plumber confirmed today that the thermostats had to be wired in separately.  he says that he has ordered two new elements and new thermostats as well so that he starts from fresh and not rely on the existing heaters.  he said they were all new so it is very puzzling why they are both faulty unless he damaged them when he fitted them and the damage is inside the tank so the electrician can not see the damage as he didn't take them out of the tank to inspect them.

If it is the thermostats, a photo of the wiring should easily confirm it.  Would be surprised if the electrician didn't pick this up though?

Jeepers_Creepers

You'll need another sparky - not the one you used before :-(

Thanks for the explanation, Mickey. I used to think it detected the actual current 'leaking' to earth, but what you explained rings a bell as I recall a sparky telling me it actually involved this 'balance' of currents, so that makes much more sense :-)

Mickey666

AskAsk said:

the electrician says that both elements are 'shorting to earth', do you know what this means?

In my book a 'short circuit' usually means a 'hard' fault such as the live and neutral conductors actually touching somewhere, somehow.  This will usually be a low resistance fault, meaning that a high current can flow, which will trip the MCB (which is an over-current' device similar to a fuse) but not necessarily an RCD.

A leakage fault is (usually) a 'soft' fault that is a high resistance, meaning that only a small current will flow to earth, which is why it is referred to 'leakage'.  This can often occur if an electrical device becomes damp or wet - the water cannot (usually) conduct enough current to trip the MCB or blow a fuse but it can conduct enough current to give someone a nasty shock . . . except if there is an RCD to trip the power when the leakage is detected before a shock is felt or can do harm.

Basically, an MCB (or a fuse) will protect against too much current flowing down the 'live' wire, no matter where that current goes, whereas an RCD will protect against too much current 'leaking' to somewhere it shouldn't (such as through a person!).

Also, in general, MCBs trip because of high currents whereas RCDs trip because of very small currents.

Mickey666

Jeepers_Creepers said:

Thanks for the explanation, Mickey. I used to think it detected the actual current 'leaking' to earth, but what you explained rings a bell as I recall a sparky telling me it actually involved this 'balance' of currents, so that makes much more sense :-)

There used to be devices called ELCBs (Earth Leakage Circuit Breakers - a variation of a Residual Current Device), which did exactly what you describe but I'm pretty sure all modern RCDs use the 'balance' method.  This is a better approach since it will also detect leakage between neutral and earth.

While RCDs are a big advance in safety compared to just an MCB or a fuse, they cannot prevent electric shocks in every possible scenario.  For example, if part of a person should ever come between the live AND neutral then that person becomes part of an electrical circuit and current will flow through them but the live and neutral conductors will (probably) remain in balance so the RCD will not trip, causing a very nasty scenario indeed.  Just one reason among many NOT to be complacent with electrical things.

There is a well known 'rule' among electrical/electronic engineers about working with high voltage equipment (typically anything between about 50 to 250 volts), which is to always keep one hand in your trouser pocket whenever possible.  The aim is to prevent a shock across the heart should both hands accidentally come into contact with live wires.

But the best approach by far is to leave such things to someone who FULLY understand the potential implications of what they're doing.

Jeepers_Creepers

Fably explained - thanks Mickey.