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After some heavy rain in Cornwall electricity does not work in the barns. This trip box

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seems at fault, though it costs 100+GBP to replace it I'm told. Can anyone give me more information on what this is?

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Call an electrician to have a look at the circuit the "trip box" is connected to. –  Tester101 Oct 27 '11 at 14:29
    
Re your flag. The image isn't hosted on Flickr. It's been copied to Imgur, so assuming this is your picture you took there's no licencing issues. –  ChrisF Oct 28 '11 at 18:44
    
It's called a residual current device or RCD and is designed to protect you in event of an electrical fault. –  Matt Jul 10 '13 at 9:25

4 Answers 4

up vote 41 down vote accepted

The advice given in the other answers is good: begin by assuming that the safety system is operating correctly. Find the fault, and hire an electrician to do so if you cannot. Only if you have evidence that there is no fault should you assume that the safety system is broken.

However, it is important to understand what this safety system is for, how it works, and what it does and does not protect.

There are a number of commonly-encountered electrical safety devices found in private residences; the three most common are:

  • ground fault protection. That's probably what this device is.

  • overcurrent protection. It is possible that this device also provides overcurrent protection, but not all ground fault protectors do.

  • arc fault protection. It is unlikely that this device is an arc fault protector.

Each solves a different problem, and you cannot rely on a device that solves one problem to solve the other unless the device is specifically designed to do the functions of two different kinds of protection.

The purpose of a ground fault protector is to prevent any current from going from hot to ground through a person, or, in the case of your barn, livestock and so on. Ground fault protection works by monitoring the current on the hot and the current on the neutral. Imagine that electricity is water; a ground fault protection device verifies that the same amount of water is going down the drain as is coming out of the faucet. If the drain is draining less than the faucet is producing, the water has to be going somewhere unexpected. If the ground fault protector detects that the difference between the hot and the neutral currents is more than, say, 30 milliAmps in your case, the hot is turned off by the box tripping.

Note that a ground fault protector provides no protection against electrocution from a hot-to-neutral connection through a person! In that case you are simply an electrical resistance like any other light bulb, and the ground fault protection will not detect the problem. If you somehow manage to touch both the hot and neutral wires of a ground-fault-protected circuit without 30 mA of the current going to ground, the device will not trip.

If your box is tripping frequently because of a ground fault then either (1) something is miswired; you have two hots sharing a neutral for example. (See below.) Or (2) something is draining current from hot to ground somewhere in the system; if you get between the hot leak and the ground, you're going to be the path to ground. Find it and fix it before that happens.

The fact that it happens more when things are wet is a sign that this is a ground fault. Something is draining current from hot to ground, but it is a good enough insulator to not trip the box when it is dry.

A ground fault protector does not protect against fires. There are two ways that electrical fires typically start: either the current is too high for a sustained period, and the wires heat up enough to burn the walls, or the current is too high for a split second, an arc is produced by the electrical outlet / the frayed lamp cord / the loose connection / whatever, and the arc makes sparks that light something on fire.

Ground fault protectors do not protect against either fire-causing failure. It is entirely possible for an electrical system to be way over-current but the current is still equal on both the hot and the neutral. And arcs might be so fast that the ground fault protector does not have time to trip before the current imbalance is restored.

Overcurrent protection by contrast typically monitors the current on the hot only. If the current on the hot exceeds some threshhold, the current is interrupted. Overcurrent protection does not protect against ground faults; if you have a 30 amp breaker and you are draining one amp of current to ground via your body, the 30 amp breaker cares not a bit. The point of the overcurrent protector is to prevent a short circuit from raising the current on the hot to the point where it could start a fire in the walls; it takes way less current than that to kill someone.

Overcurrent protection also does not monitor current on the neutral. The current on the neutral and the current on the hot are supposed to be equal -- recall our water analogy. But imagine you had two faucets pouring into one drain. The current on the two hots might be below 30 amps, but the combined current on the neutral might be higher. Just because the neutral doesn't have voltage on it does not mean it does not have current. It can still get hot enough to burn. Never share a neutral between two hots; that's an "Edison circuit" and they are not safe. (And they will not work with a GFCI, obviously, since the currents on the hot and the neutral are almost never equal if any current is flowing.)

Similarly, arc fault protection does not protect against you being the path to ground for a low-but-deadly current, or sustained high currents; it protects against transient high currents producing arcs that could set nearby paper or carpet or mouse nests or whatever on fire.

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You're going to be a great asset here. :) –  The Evil Greebo Oct 27 '11 at 18:57
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TL;DR version... In a typical residence: "circuit breakers" protect against fires in walls, AFCIs protect against fires near walls, and GFCIs protect against me blow-drying my hair in the shower. Correct? –  hemp Oct 27 '11 at 19:50
    
@hemp: You got it. –  Eric Lippert Oct 27 '11 at 20:36
    
For some reason, I got involved a little: as marked in the photo, this is a BS4293 RCCB which apparently is a "Residual Current operated circuit breaker", and looks like it is "obselete" (the lagrange online catalog does not have this by part no.) ( see for example: theiet.org/forums/forum/… ) also it does not look like it has overload protection, as that would be labeled RCBO (?) –  horatio Oct 27 '11 at 20:36
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After some trial & error my neighbour discovered that our shredder was causing it to trip. The problem was fixed by rewiring the shredder. Thank you for some education! :) –  Kai Hendry Oct 29 '11 at 12:36

Yes those are very expensive- but who ever told you to replace that is incorrect .. very incorrect! You see what is written on there - "TRIP 30mA" that means that when ever it senses 30mA between N and Ground(Earth) difference it means there is a leakage of electricity. It is most likely into the ground but it depends where this Earth Leakage Breaker is.

Is it in your house and the wire goes to the barn an distance away? Or is it in the barn?

If it is in the barn it means there is some dampness building up somewhere- junction box most liekly causing a micro short. This device most likely is saving your barn from burning down! Do not ignore that. To find the fault will be difficult and will mean isolating parts of the circuit to track down the offending device/junction box.

If it is in your house.. then you just need to include the possibly that there is fault in the barn like i mentioned higher AND/OR in the cable going from your home to the barn.

Either way you might have to spend some cash- but not on replacing that unit! Maybe on an electrician!

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Hold on a moment here. The point of a ground fault breaker is not to prevent the barn from burning down. You should not rely upon a ground fault breaker to prevent an overcurrent situation. The point of a ground fault breaker is to prevent humans from being electrocuted. Your conclusion is correct: the guy needs to find the fault and fix it. But not to save the barn from burning down; to keep humans or animals from becoming the path to ground. –  Eric Lippert Oct 27 '11 at 17:13
    
@Eric Although there is at least one error in ppumkin's answer (GFCI breakers trip if there is a difference in current between line & neutral, not neutral & ground,) he/she is correct in saying that a GFCI breaker can prevent fires. A normal breaker provides overcurrent protection, and a GFCI breaker does both overcurrent protection and personal protection. I'm not familiar with the device in question, and it's possible that this specific device doesn't provide overcurrent protection, but a typical GFCI breaker in a service panel will. –  Michael Oct 29 '11 at 12:40
    
@Eric Upon reading your answer, I can see that you realize that GFCI breakers also provide overcurrent protection: "It is possible that this device also provides overcurrent protection, but not all ground fault protectors do." The use of "breaker" in your above comment is perhaps the wrong term to use, as a "breaker" is an OCP device as defined by NEC. "Stand-alone GFCI device" would perhaps be a more precise term to use in order to differentiate it from a GFCI breaker. –  Michael Oct 29 '11 at 13:01
    
@EricLippert Yes- but given the situation when a rare but possible event of ground leakage occurs above the wires rating over a longer period making the wire hotter and hotter(with out tripping the over current breaker) - it can cause a fire. The ground leakage does not necessarily have to be human/animal. I over shot the definition of the ground breaker- yes I agree. This is circumstantial and happens with bad installations, usually 1 in millions..in a world of filled with billions of people.... –  ppumkin Nov 1 '11 at 17:06

Sounds to me like the trip box is doing its job - which is to shut down power when there's a short in the lines.

Examine carefully your circuits for any water getting into the lines which are causing the real problem.

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The other answer said that ground fault interruptors are to prevent fires, and you're saying that they are to prevent shorts. Neither is the case. Ground fault interruptors are to prevent ground faults, not short circuits. –  Eric Lippert Oct 27 '11 at 17:14
    
Ok that's a fair point - a ground fault is different from an over-current situation. however the box in question trips at 30ma - which suggests this device is protecting against both cases. –  The Evil Greebo Oct 27 '11 at 17:31
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It trips when there is 30mA difference between the hot and neutral. It doesn't care what the load is. The only thing I can think of that takes less than 30mA, that you would use with line voltage, is a simple test light. –  Brad Gilbert Oct 27 '11 at 23:33

It sounds like something is getting wet, as all the wiring is protected by a single RCD, is will be very hard to find out the problem.

For some reason there is more than 30mA difference between the current in live then natural. Given that some devices “leek current” anyway, the fault could be a lot less than 30mA, or there could be more than one circuit with a fault that add up in total to more than 30mA. It may show up with testing by an electrician if you have very good lack.

Try turning of the MCB that covers any outside lights (and anything else outside), and see if the trips stop, if so you know where to start looking for the problem.

You may wish to replace your consumer unit and have a separate RCBO for each circuit; this will lead to only a single circuit being tripped and a lot easier fault finding job. (I am very glad we paid for this more expansive option when we had our consumer unit brought up to current rags. RCBOs do not cost much more than MCBs, but most electricians try to charge a lot more for them)

A cheaper option would be to fit a consumer unit with 2 RCDs, and then put a single circuit on one of the RCDs, keep moving the circuit over to that RCD until it become clear witch circuit has the fault. (It is not safe for most DIY people to move circuit in a consumer unit, as part of the consumer unit will always be live)

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