# Can I use a 100mA RCD?

## Can I use a 100mA RCD?

A 100 mA RCD is not to be used for personal protection, it needs 30mA for this to the regulations in domestic properties, and I would also recommend it in this case, for commercial properties.

### What is the maximum permitted Zs for a 30mA RCD?

1667 ohms
Protection by 30mA RCD Table 41.5 allows a maximum permitted Zs of 1667 ohms for a 30mA RCD although Note 2 to Table 41.5 states that a value exceeding 200 ohms may not be stable. Appendix 3, Table 3A of BS 7671 shows that a 30mA general, non-delay RCD will operate within 300mS (0.3s).

#### Is a 100mA RCD more sensitive than a 30mA?

Yes, 30mA is safer than 100mA. 10mA, the lowest standard size, is safer still.

How do you calculate Zs?

Therefore, where reliable measured values are available for the external earth loop impedance (Ze) and for the loop resistance of the line and protective conductors (R₁ + R₂) of the circuit, it is permissible to derive the loop impedance of a circuit by using the following formula: Zs = Ze + (R₁ + R₂).

What code is a high Zs reading?

The latest PIR shows a code 2 for the high Zs. If a Zs value is high, then you need to establish whether it is due to circuit conditions, supply conditions, or a fault. You can then descide the appropriate course of action.

## What is the maximum tripping time for a 100mA RCD?

In the current Regulations, the maximum time allowed is 1s, in the 16th the time was 5s. The tripping current should be no greater than 5x the rated operating current. As such a 100mA RCD should trip within 1s (5s for 16th) at a test current no greater than 500mA.

### Can RCD be used to compensate for high ZS?

Using an RCD to compensate for a high Zs in a TN system, whilst being a sensible precaution (failure 7%?) and is allowed in TT systems anyway as no other option. I would be asking why the high Zs.

#### What does RCD protected mean on a circuit?

if a circuit is a bit over its max zs but is rcd protected does this make it still safe and to regs with the max being 1667ohms? thanks The RCD ensures your circuit meets the requirements for disconnection times, (as on most TT installations).

What happens if the ZS of a circuit is too high?

If Zs is that high then there is inadequate earth fault current to disconnect a circuit in defined time.

Do I need an RCD for ads protection?

Having an RCD there (say for additional protection) doesn’t necessarily mean it’s being relied on for ADS (protection against indirect contact). Many people prefer to use overcurrent devices (better reliability, both from failures of the device and also failures in supply) than RCDs, so will expect MCB/fuse Zs values even if an RCD is present.