ELCB too sensitive 2

[matwong]

This is one of the common question I always encountered but without any firm answer.

There is always a particular house among a row of link-houses which ELCB operates whenever there is thunderstorm weather (whereas the rest of the houses are intact). All the houses having the same rating of ELCB which is 100ms. Any idea what is the cause of this nuisance tripping and how to solve it ?

What I can think of is that the particular link-house has better earthing than the rest of link-houses but this doesn't offer any solution (I'm sure none of us will agree to reduce the effectiveness of the earthing). Some people suggested to increase the sensitivity of ELCB to 300ms but why ? What is the technical justification ? I'm more incline to have better solution.

From my observation, there is no pattern of which link-house affected, sometime at the corner lot and sometime can be at the middle one. These link-house normally shared the common mains wiring which is tapped from a single point of 415V overhead lines (TT & multiple earthed system).

 

[digitrex]

Sensitivity of ELCBs should be in mA, not ms.

To check nuisance tripping of ELCB, I always do in the following sequence:

1. Somewhere inside the house may have a little earth leakage that has not reach upto the tripping level. A lightning is just a little surge to add into the leakage. You need to have all the house wirings checked. You may test trip the ELCB using a RCD tester that plugs into an unused socket outlet. The best RCD tester is the model with AUTO RAMP function (

http://www.kew-ltd.co.jp/en/products/rcd/5406A.html)

which you can measure the additional leakage current required to trip the ELCB.

2. Replace the ELCB by the same rating of another brand (there are some good models that can withstand transients). Changing to higher sensitivity is a stupid thing to do, instead, you should consider a more sensitive (30mA) ELCB, for human life protection.

 

[ScottyUK]

RCD's are often quoted as having a mA rating and a ms rating. Time delayed RCD's are used to provide discrimination in TT installations with a split load board where a 30mA instantaneous RCD provides personnel protection for socket outlets and a 100mA time delayed RCD provides ground fault protection to ensure fault clearance within the 5s required for fixed equipment. Apologies for quoting the UK regs, but this looks like a UK installation based on voltages. ELCB is an old term, incidentally.

Check the TT earthing arrangement at each property - you will need an earth tester to make meaningful readings. If all looks well, Digitrex' suggestion to replace the RCD with a different brand is not unreasonable. I would not change the ratings - 100mA / 100ms is an entirely reasonable spec for an incoming main protective device in a TT installation, but I would like to think that there was a secondary 30mA / instantaneous RCD provided for personnel protection where appropriate.

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I don't suffer from insanity. I enjoy it...

 

[gary071]

ScottyUK,
Sorry that i'm not familiar with the TT earthing arrangement.So, can u give more detail on how to check the TT earthing arrangement in order to solve the problem?And what readings should be obtained in earth tester to make sure the earthing arrangement is in good situation?

Very much appreciate your comments.Thanks.

 

[ScottyUK]

A TT, or Terra-Terra, earthing arrangement uses the bulk of earth to provide the return path to the distribution transformer rather than a copper conductor. The transformer would be grounded to a buried earthing electrode or mat. At the customer's premises an earth electrode - typically a driven rod - is provided. The soil conditions are highly significant in determining the earth loop impedance - a dry sandy soil will provide a far higher earth resistance than a similar electrode in a damp loam soil, for example.

Earth loop impedance is an important parameter to consider when determining how fast a breaker or fuse will clear a fault. The maximum times will be quoted in the local regulations or code. In the UK the maximum times are 400ms for a circuit feeding socket outlets and certain other items. Circuits feeding 'fixed equipment' have a maximum clearing time of 5s. Given those requirements, there are a couple of options open: ensure that the earth loop impedance is low enough to ensure that the overcurrent protective device - fuse or circuit breaker - operates within the required time, or use a supplementary device such as an RCD to ensure rapid earth fault clearance. The former option can be really difficult to guarantee with a TT system under all conditions if you factor in dry weather etc, so the second option is more common.

The required earth loop impedance, and by implication the earth electrode impedance to the bulk of earth, really depends on the configuration of the installation. With an RCD as the main incoming device it is possible to have a fairly high earth resistance and still comply with the regulations; without an RCD the earth impedance must be much lower. Without all the system details it isn't possible to give a definite answer on an acceptable earth electrode resistance because this value interacts with other circuit parameters and influences choice of components. In the UK the maximum voltage present on the earth electrode must not exceed 50V in normal premises, so in theory you could have an earth electrode resistance of just over 1.6k[Ω] using a 30mA RCD. Without the RCD it would be just a few ohms. There are few tables toward the bottom of the link below which give some idea of the acceptable earth loop impedances for various breakers and fuses:

http://www.tlc-direct.co.uk/Book/5.3.4.htm

The following link will give you some idea of how testing can be performed, as will some of the manufacturer's websites.

http://www.tlc-direct.co.uk/Book/8.6.1.htm

The whole website is worth a poke about on - obviously I haven't checked everything, but there are some useful bits of information in there.

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I don't suffer from insanity. I enjoy it...