Difference between revisions of "RCD"

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'''Residual Current Devices''' or '''RCDs''' are used for protecting circuits against faults causing current to flow to earth, and in some cases protecting users from electric shock.
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Work in progress
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A '''Residual Current Device''' or '''RCD''' is a circuit protective device designed to protect users from electric shock. They are also used in any circumstance as a circuit protective device where it is not possible to achieve normal operation of traditional protective devices (Fuses, MCBs), due to earth loop impedances to high to ensure correct operation of the fuse or MCB.
 +
 
 +
=What Does it do?=
 +
A RCD detects a fault condition which would be typically only be seen on a live circuit, when a person is receiving an electric shock from the circuit. When this situation is detected it automatically isolates the supply to the circuit (or group of circuits).
 +
 
 +
This gives greatly enhanced shock protection for both direct contact (i.e. contact with an exposed live wire - say touching a flex you have just damaged with a power tool), and indirect contact (e.g. when the metal casing of an appliance becomes live due to an internal fault) faults. RCD protection is particularly important to protect users in high risk locations where they may be more susceptible to electric shock such as bathrooms, pool areas, saunas, or simply when using power tools or appliances outside (basically anywhere the user can be expected to be wet, barefoot, or in good contact with earth.
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 +
=What does it '''not''' do?=
 +
RCDs do not offer any overcurrent protection, and hence they will not clear short circuits or faults that result in an appliance drawing excessive current. When using a RCD protected supply to feed a power tool, they will offer no protection should you make contact with another live non protected circuit with the tool.
 +
 
 +
=How does it work?=
 +
RCDs are current balance devices. They measure any current imbalance in the flow in and out of a circuit or appliance via its Live and Neutral conductors. Should the current imbalance exceed the tripping threshold for the device, it will activate and disconnect the circuit.  
  
 
For more information see http://en.wikipedia.org/wiki/Residual-current_device
 
For more information see http://en.wikipedia.org/wiki/Residual-current_device
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 +
=Types of RCD=
 +
 +
There are a umber of form factors and technical ratings associated with RCDs. Selecting the correct unit for the job in hand is critical.
 +
 +
==Enclosure design==
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{| border="1" cellpadding="6" cellspacing="0" style="text-align:left"
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|-
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!Type
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!Description
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|-
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| Integrated plug or Socket || There are a range of RCDs that are built into plugs and sockets. These are designed to offer enhanced shock protection to either an individual device or small number of devices. Typically found on extension leads. There are also some fixed wiring sockets that include RCD protection, again designed to provide a safer connection point for certain categories of appliance.
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|-
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| Integrated RCD Spur      || An RCD integrated into a spur connection unit. Designed to provide individual RCD protection any fixed equipment that has a high electrical shock risk (e.g. a pool/bath hoist for disabled access).
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|-
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| Standard DIN rail mounting || This is a modular device in a standard form factor that is designed to be used in electrical enclosures such as consumer units and other similar enclosures. These RCDs typically occupy two module widths (i.e. the space taken by two MCBs), and can be sued to power one or more circuits.
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|}
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 +
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==Electrical and Trip Characteristics==
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{| border="1" cellpadding="6" cellspacing="0" style="text-align:left"
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|-
 +
!Type
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!Description
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|-
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| Rated Current || This is the maximum current the device is rated to carry. Commonly used domestic devices in DIN rail mounts are commonly available in 63A and 80A ratings. 
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|-
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| Number of poles  || RCDs are available for protecting both three phase and single phase circuits. (Three phase devices being typically twice the width of single phase ones)
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|-
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| Trip threshold or sensitivity || This is the maximum current imbalance that will be tolerated without the trip mechanism being activated. In reality the devices specifications are usually scoped such that the device will trip on 66% of the rated trip current (so as little as 20mA may be required to trip a 30mA device). Common trip thresholds include:
 +
* 10mA
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* 30mA
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* 100mA
 +
* 300mA
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|-
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| Trip time  || General purpose RCDs (sometimes marked with a "G") are designed to trip as soon as possible after a trip condition is detected, and at any rate within two cycles of the mains (40mS for UK 50Hz supplies). There are also time delayed types that are designed to trip only after exposure to a trip fault condition that lasts longer than a pre-set delay (typically two seconds). The time delayed type (of denoted with a "S" suffix) are particularly useful where it is required to cascade RCDs. The time delay maintains discrimination between the cascaded devices so that the once closest to the fault trips first.
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|}
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TBC
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Notes on stuff to come
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Types / applications
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 +
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two main uses, reducing shock risk, coping with high Zs.
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 +
 +
Nuicense trips
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 +
trip mechanisms
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  high leakage
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  sensitising
 +
  current surge
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  N E Shorts (may only realise on surge)
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 +
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tracing the cause
 +
 +
Empirical Tests:
 +
 +
The likely suspects
 +
  heater appliances, damp, outside electrics
 +
Isolate circuits
 +
Disconnect accessories from circuits
 +
Introduce known leakage currents to a circuit
 +
 +
 +
Measurement Tests
 +
 +
DC Resistance
 +
Insulation resistance
 +
High sensitivity clamp meters
 +
series earth current measurements
 +
 +
  
  

Revision as of 15:33, 25 May 2007

Work in progress

A Residual Current Device or RCD is a circuit protective device designed to protect users from electric shock. They are also used in any circumstance as a circuit protective device where it is not possible to achieve normal operation of traditional protective devices (Fuses, MCBs), due to earth loop impedances to high to ensure correct operation of the fuse or MCB.

What Does it do?

A RCD detects a fault condition which would be typically only be seen on a live circuit, when a person is receiving an electric shock from the circuit. When this situation is detected it automatically isolates the supply to the circuit (or group of circuits).

This gives greatly enhanced shock protection for both direct contact (i.e. contact with an exposed live wire - say touching a flex you have just damaged with a power tool), and indirect contact (e.g. when the metal casing of an appliance becomes live due to an internal fault) faults. RCD protection is particularly important to protect users in high risk locations where they may be more susceptible to electric shock such as bathrooms, pool areas, saunas, or simply when using power tools or appliances outside (basically anywhere the user can be expected to be wet, barefoot, or in good contact with earth.

What does it not do?

RCDs do not offer any overcurrent protection, and hence they will not clear short circuits or faults that result in an appliance drawing excessive current. When using a RCD protected supply to feed a power tool, they will offer no protection should you make contact with another live non protected circuit with the tool.

How does it work?

RCDs are current balance devices. They measure any current imbalance in the flow in and out of a circuit or appliance via its Live and Neutral conductors. Should the current imbalance exceed the tripping threshold for the device, it will activate and disconnect the circuit.

For more information see http://en.wikipedia.org/wiki/Residual-current_device

Types of RCD

There are a umber of form factors and technical ratings associated with RCDs. Selecting the correct unit for the job in hand is critical.

Enclosure design

Type Description
Integrated plug or Socket There are a range of RCDs that are built into plugs and sockets. These are designed to offer enhanced shock protection to either an individual device or small number of devices. Typically found on extension leads. There are also some fixed wiring sockets that include RCD protection, again designed to provide a safer connection point for certain categories of appliance.
Integrated RCD Spur An RCD integrated into a spur connection unit. Designed to provide individual RCD protection any fixed equipment that has a high electrical shock risk (e.g. a pool/bath hoist for disabled access).
Standard DIN rail mounting This is a modular device in a standard form factor that is designed to be used in electrical enclosures such as consumer units and other similar enclosures. These RCDs typically occupy two module widths (i.e. the space taken by two MCBs), and can be sued to power one or more circuits.


Electrical and Trip Characteristics

Type Description
Rated Current This is the maximum current the device is rated to carry. Commonly used domestic devices in DIN rail mounts are commonly available in 63A and 80A ratings.
Number of poles RCDs are available for protecting both three phase and single phase circuits. (Three phase devices being typically twice the width of single phase ones)
Trip threshold or sensitivity This is the maximum current imbalance that will be tolerated without the trip mechanism being activated. In reality the devices specifications are usually scoped such that the device will trip on 66% of the rated trip current (so as little as 20mA may be required to trip a 30mA device). Common trip thresholds include:
  • 10mA
  • 30mA
  • 100mA
  • 300mA
Trip time General purpose RCDs (sometimes marked with a "G") are designed to trip as soon as possible after a trip condition is detected, and at any rate within two cycles of the mains (40mS for UK 50Hz supplies). There are also time delayed types that are designed to trip only after exposure to a trip fault condition that lasts longer than a pre-set delay (typically two seconds). The time delayed type (of denoted with a "S" suffix) are particularly useful where it is required to cascade RCDs. The time delay maintains discrimination between the cascaded devices so that the once closest to the fault trips first.


TBC

Notes on stuff to come

Types / applications


two main uses, reducing shock risk, coping with high Zs.


Nuicense trips

trip mechanisms 

 high leakage
 sensitising
 current surge 
 N E Shorts (may only realise on surge)


tracing the cause

Empirical Tests:

The likely suspects 

 heater appliances, damp, outside electrics

Isolate circuits Disconnect accessories from circuits Introduce known leakage currents to a circuit


Measurement Tests

DC Resistance Insulation resistance High sensitivity clamp meters series earth current measurements



Nuisance Trips

One problem with RCDs is nuisance trips. Nuisance trips are when an RCD cuts the power without good reason, ie when no safety risk is present.

There are a number of reasons why these trips occur, and the problem can not be entirely eliminated. Certain strategies are therefore wise to reduce both the occurrence and the consequences of nuisance trips. These are described in the Rewiring Tips article in the following sections:


RCDs and drills

DIYers sometimes put an RCD plug onto their mains drill in the belief that it will stop them being electrocuted if they drill into a cable. In fact it offers no such protection, and tends to encourage less care, resulting in greater danger.

An RCD on the drill plug does not detect any current flowing from wall cable to drill body to user. Nor can it switch this current off. It thus has no effect on such shock scenarios.

An RCD plug on a drill can increase risk more than it reduces it by providing a false sense of security.


See also

ELCB

My RCD Trips

Wikipedia

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