http://www.westernautomation.com/pages/demystify.htm
Problems With RCDs
As far as users are concerned, the single greatest problem with RCDs is
nuisance tripping. This problem will either be due to the design of the RCD
or due to the installation. Nuisance tripping can be a frustrating problem
for the user and there have been cases where RCDs were strapped out, removed
from installations completely or replaced with RCDs with a higher rated trip
current. All of these types of actions are very dangerous and should be
avoided. Where the problem lies with the RCD design, it is usually
attributable to factors such as voltage spikes, surges, switching
transients, noise, inrush currents, etc. Over recent years, the problem of
nuisance tripping attributable to the RCD itself have been addressed by IEC.
New tests have been introduced into the product standards to ensure that
RCDs have a reasonably high immunity to nuisance tripping. Unfortunately,
all RCDs do not comply with the new requirements. To minimise this problem,
ensure as far as possible that the RCD has compliance to IEC61008 or EN61008
(RCCBs) or IEC61009 or EN61009 (RCBOs) and also to IEC61543 or EN61543 (EMC
requirements for RCDs).
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Sometimes the problem of nuisance tripping is attributable to the
installation. IEC recommend that the standing earth leakage current on the
installation should not exceed 30% of the rated trip current of the RCD
intended to be used on that installation. This means that for a 30mA RCD,
the standing earth leakage current should not exceed 10mA. Given that a 30mA
RCD may trip anywhere from 15 - 30mA, a 10mA standing leakage current will
virtually prime the RCD to trip. In general, RCDs cannot tell the difference
between a standing leakage current and an earth fault current. The sum of
these two currents is the residual current seen by the RCD and if this
aggregate current is greater than its rated trip current, the RCD will trip,
unless it's faulty. (but that's another story)
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The problem of standing leakage currents is increasing. This is due to two
factors:
1.. equipment manufacturers filtering internally generated noise to earth
2.. fitting of RFI suppression to provide immunity to mains borne noise.
Recently introduced European Directives on EMC require manufacturers of
products, appliances, equipment, etc. to contain within specified limits the
levels of RFI type emissions produced by their products. Manufacturers often
have to resort to the use of filtering circuits to meet these requirements.
The filtering circuits can divert high frequency signals to earth, but can
also result in the flow of significant levels of leakage current to earth at
the standard 50Hz frequency. Under IEC rules, electrical appliances may
allow a standing leakage current of up to 3.5mA/50Hz to flow to earth. Such
current levels coupled with other sources of standing leakage currents can
give rise to nuisance tripping.
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Washing machines, oil or gas burners and even immersion heaters can
contribute to nuisance tripping. Temperature and humidity may also be
factors in nuisance tripping. Other factors contributing to nuisance
tripping may be poor earth terminations, surge suppressers, neutral voltage
rising above earth potential, etc. A very sneaky problem is where an earth
fault current on the supply side of the RCDs manages to appear as a load
side fault to the RCD, causing it to trip. This problem is more usually
associated with IT systems, or on TT systems with high earth impedance.
Before changing the RCD in response to a nuisance tripping problem, check
the installation by measuring the standing leakage current and carrying out
an earth loop test. Also try to identify equipment that is likely to
contribute to standing or transient earth currents. A residual current
monitor can be fitted to an installation to detect the level of the standing
earth leakage current or even transient leakage currents.
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VD - v - VI RCDs
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[george]
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