Nuisance tripping RCD

Help Support Talk Electrician Forum:

SparkyMarky

Member
Joined
Apr 28, 2008
Messages
41
Reaction score
0
This is not one of my jobs. Domestic. TNC-S. Split load board. 80A 30mA RCD nuisance trips as do several others on the same estate. No obvious link to fauly appliance, faulty circuit etc. Installation checked out by local electrical firm. Their solution was to replace the RCD with a 100A 100mA time delay RCD. Problem solved. My understanding is that this does not conform to regs as ground floor sockets should have 30mA, 40 ms protection so suggested that could use 30mA RCBO for ground floor ring. Southern Electric are involved as several houses on the estate have the same problem and today their guy said that the time delay RCD was fine but be sure to use 30mA RCD plugs for outdoor equipment.

So, founts of all knowledge electrical, what do the regs say? (Steptoe, don't start your reply with " if you have to ask this blah blah...")

 

Mr Sworld

Electrical Technician
Joined
Mar 21, 2008
Messages
1,152
Reaction score
0
100mA and 500mA rcd's are there to protect equipment not you. However as they are time delays they have put in just swap any power circuit mcb's that could power outside equipment with 30mA RCBO's and you'll comply (at present!). :D

Haven't got a copy of the regs with me (it's 'down time') but that will satisfy.

 
Joined
Mar 7, 2008
Messages
9,994
Reaction score
0
Location
In the office again.
This is not one of my jobs. Domestic. TNC-S. Split load board. 80A 30mA RCD nuisance trips as do several others on the same estate. No obvious link to fauly appliance, faulty circuit etc. Installation checked out by local electrical firm. Their solution was to replace the RCD with a 100A 100mA time delay RCD. Problem solved. My understanding is that this does not conform to regs as ground floor sockets should have 30mA, 40 ms protection so suggested that could use 30mA RCBO for ground floor ring. Southern Electric are involved as several houses on the estate have the same problem and today their guy said that the time delay RCD was fine but be sure to use 30mA RCD plugs for outdoor equipment. So, founts of all knowledge electrical, what do the regs say? (Steptoe, don't start your reply with " if you have to ask this blah blah...")
I am having a think as to how a fault on a pme supply can affect the RCD`s. Leave that with me; unless someone else has a quicker brain than I ( quite likely)

As regards the uprated RCD; it does NOT meet regs (16th OR 17th). A 30mA time delay would be ok; but not 100mA.

The RCBO would be subject to the same problem as the RCD`s are experiencing; therefore that`s a non-starter.

I`ll be back. After. When I`ve had a thunk

 

SparkyMarky

Member
Joined
Apr 28, 2008
Messages
41
Reaction score
0
So are you saying that on this occasion the local electrician and Southern Electric are wrong and, dare I say it, I'm right?

 

SparkyMarky

Member
Joined
Apr 28, 2008
Messages
41
Reaction score
0
KME are you sure a 30mA time delay would be OK as I thought that there had to be a 40ms disconnection time for supplementary protection?

 

SparkyMarky

Member
Joined
Apr 28, 2008
Messages
41
Reaction score
0
Regs in back pocket, as ever! How does one know that a 30mA time delay RCD will disconnect within 40 ms at 5 x In? I thought time delay RCD's were slugged to allow downstream RCD's to react first ie within 40ms. So how can a time delay conform in this situation?

 
Joined
Mar 7, 2008
Messages
9,994
Reaction score
0
Location
In the office again.
Ignore my previous post. "s" type will trip at40 mS @5In; BUT isn`t readily available as a 30mA device. However:

DETAILED FAULT-FINDING ON

RCD-PROTECTED INSTALLATIONS

An RCD will detect and trip not only on a phase-to-earth fault but also on a neutral-toearth

fault. The majority of earth faults occur in appliances, particularly portable appliances

and their flexible cables. This means that in many installations, faults can be located easily

by unplugging appliances until the RCD stops tripping. However, it is not uncommon to find

a floorboard nail driven between the neutral and earth conductors, reversed neutral and

earth connections or a neutral conductor touching an earthed mounting box. It should

also be remembered that withdrawing a fuse or tripping a circuit-breaker in a final circuit

does not normally interrupt the neutral and may not prevent an RCD from tripping. Such

a condition could occur during a partial re-wire where the RCD is already installed.

Cutting through a cable could cause the RCD to trip but this may not be noticed at the

time. During fault-finding, the trip may not be associated with the cutting of the cable.

The most effective way of testing for earth faults in the wiring or equipment is by measuring

the insulation resistance from phase to earth and from neutral to earth (as described in BS

7671 Section 713-04.) It is essential to ensure that the electrical circuit is isolated from

the mains. It is also important to ensure that there are no time-switches, connectors etc.

isolating any part of the circuit from the test equipment whilst the tests are carried out.

Care should also be taken to ensure that there is no equipment that will be damaged by

the tests.

Even when equipment cannot be unplugged, a phase-to-earth fault is relatively easy to find

since each phase conductor can be isolated by withdrawing the fuses or by tripping each

circuit breaker. The faulty sub-circuit can then be identified by replacing each fuse or closing

each circuit-breaker in turn until the RCD trips.

In the case of a neutral-to-earth fault, all neutral conductors must first be disconnected

from the neutral bar. The faulty sub-circuit can then be identified by connecting each

neutral conductor in turn onto the neutral bar until the RCD trips.

The above discussion only applies if the phase-to-earth or neutral-to-earth fault is of low

enough impedance to allow sufficient earth fault current to flow to trip the RCD.

It might be assumed that any standing protective conductor current below the trip level of

the RCD could be ignored. Unfortunately this is not so because the RCD sensitivity is

effectively increased to the difference between the RCD trip current and the standing

protective conductor current. For example, an RCD with a rated residual operating

current of 30mA will have a typical trip current of 22mA; if the standing protective

conductor current is 10mA it will only take an earth fault current of 12mA to trip the RCD.

This could lead to unwanted tripping.

It may be possible to obtain an approximate measurement of this standing protective

conductor current in domestic, commercial and some light industrial installations simply by

connecting a milliammeter into the main earthing conductor. Care must be taken while

performing this test since exposed and extraneous metalwork could become live if the

main earthing lead is disconnected and a phase-to-earth fault exists. Safety is paramount

and the contractor must ensure that the consumer will not come into contact with any

exposed and extraneous metalwork while the test is being carried out.

However, this method is unlikely to work where the means of earthing is provided by the

electricity supplier and the supply is PME or

 
Joined
Mar 7, 2008
Messages
9,994
Reaction score
0
Location
In the office again.
part 2:

Experience in the field has shown that tripping due to surge currents is not the major cause

of unwanted tripping.

9.1.2. Tripping due to transient overvoltage and capacitance to earth

A transient overvoltage can be defined as a temporary surge, of limited energy, caused by Some types of discharge lighting (e.g. high-pressure sodium lamps and metal halide lamps)

use external igniters, which produce a series of high voltage pulses, which cease when the

lamp starts. These pulses are of short duration but range from 3kV to 4.5kV for highpressure

sodium lamps. Metal halide lamps are ignited by applying 9kV pulses at 10ms

intervals for up to 7 seconds directly on to the lamp. The very large number of discharge

lamps in use (particularly for street lighting) makes it likely that this is a major source of

transient overvoltages.

From the foregoing it will be seen that unwanted tripping may be caused by transient

overvoltages in the mains supply, originating from outside the installation.

The question of how these transient overvoltages trip an RCD has not yet been discussed.

Transients can appear in three possible forms:

 

Admin1

Forum Founder
Joined
Feb 6, 2008
Messages
10,278
Reaction score
-1
That reminds me KME - Extend the bandwidth for the forum.

I thought Spec had posted that one, for a minute. :^O

 
Joined
Mar 7, 2008
Messages
9,994
Reaction score
0
Location
In the office again.
er... part 3:

The old practice of protecting a whole distribution board by

a single high-sensitivity RCD can, in many cases, lead to unwanted tripping, particularly in

industrial environments where inductive loads will cause greater transient overvoltages and

where longer cable runs will result in larger values of capacitance to earth. There have been

instances of a contractor supplying a 12-way TP and N distribution board, via a 30mA RCD,

to control banks of fluorescent fittings and then wondering why the RCD kept tripping out!

Apart from initial current surges on switch-on and possible flashover due to a faulty fitting,

an isolated fluorescent fitting by itself does not appear to be a major source of unwanted

tripping. Banks of fluorescent fittings used in conjunction with other appliances will,

however, result in an accumulation of standing protective conductor current and will

present a low impedance path to earth for transient overvoltage currents through the

capacitance to earth.

9.3 Double-Pole Switching

Double-pole switching within the fixed wiring is known to produce a strange phenomenon

whereby switching OFF a double-pole switch supplied through an RCD can cause the RCD

to trip. Single-pole switching does not produce this effect, and it is known that changing

over from double-pole to single-pole switching can overcome the problem, where such

replacement is permissible and safe.

The phenomenon is explained by the fact that while capacitance between neutral and earth

will exist in all installations, the earth leakage through this capacitance will be negligible due

to the low (almost zero) potential between neutral and earth. When the neutral pole of a

double-pole switch is opened, the voltage across this capacitance will suddenly increase,

with a subsequent increase in neutral-to-earth capacitive earth leakage currents. This

increase will be at a maximum if the neutral opens first and will be aggravated by arcing at

both switch contacts which will cause high frequency voltage spikes to cause even higher

neutral-to-earth leakage currents.The effect may be aggravated further by the slow-break

feature of the switch often found in domestic a.c. switchgear.

Closing a double-pole switch may also trip an RCD (as may closing a single-pole switch),

but in this case any tripping is unlikely to be caused by neutral-to-earth capacitance since

the fast-make characteristic of the switch reduces both arcing and the time that any

significant voltage might exist between the load-side neutral and earth. Any circuit, which

incorporates only single-pole switching, will never experience this type of problem since

(under normal operating conditions) the neutral-to-earth voltage is so low that the neutral-to-earth capacitive earth leakage is negligible.

The problem appears to occur most frequently in installations where a separate consumer

unit and RCD are installed. The opening of the consumer unit switch will allow a higher

voltage (possibly 230V) to appear suddenly across the neutral-to-earth capacitance of the

whole installation. This can result in sufficient earth fault current to trip the RCD.

9.4 Cables and Overhead Lines

There are indications that the problem of unwanted tripping occurs more frequently in

installations supplied by overhead lines than by those supplied by underground concentric

cable. This can be explained by examining the capacitance and inductance of these two

types of conductor as shown in Annex 10.2. The analysis shows that:

 
Joined
Mar 7, 2008
Messages
9,994
Reaction score
0
Location
In the office again.
Sorry for the amount of data posted; but if I tell you I`ve LOTS more pages on this........

If you want further info, pm me your email; and I`ll send you the whole RCD handbook; as a PDF.

That goes for anyone else. If you want it, let me know.

Martyn

 

Admin1

Forum Founder
Joined
Feb 6, 2008
Messages
10,278
Reaction score
-1
Mart - Why don't you put the link to the actual PDF file on here, Mate.

 

Mr Sworld

Electrical Technician
Joined
Mar 21, 2008
Messages
1,152
Reaction score
0
My eyes are bleeding! So much block typing! :_|

 

Admin1

Forum Founder
Joined
Feb 6, 2008
Messages
10,278
Reaction score
-1
Eh?????X(X(X( X( Watch it.................. Watch it............... X( ]:) ] :) ]:)

someone pour water over KME's fingers...

I think I can see smoke comming off them:O:O:O
:^O Aww Bless.

 
Top