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Surge Protection Devices

trebor12345 · 2026-02-20T16:26:11Z

This Topic has been changed by the Moderators in order to address only the fitting of Surge Protection Devices (SPDs) to consumer units. The Original Post, which follows here, concerned the type of protection device which supplied a heat pump.That portion of the discussion has been deleted as it gave incorrect information on the subject of electrical safety devices. I am having my existing heat pump changed to a Vaillant (another story). The question of fuse type has come up in conversations. According to the Vaillant spec I need this: The current fuse I have is this: Is this the right one?

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Air Source Heat Pumps (ASHPs)

Last Post by Batpred 4 months ago

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Batpred

(@batpred)

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Joined: 2 years ago

Posts: 1159

12/06/2026 3:23 pm

Posted by: @transparent

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Posted by: @batpred

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So would the presence of DC components require RCDs to be fit?

Erm... I can interpret that question in more than one way, so I'm unsure of the angle you're taking.

All Type-B RCDs and RCBOs will correctly trip ii there is an element of Direct Current superimposed onto the mains 50Hz waveform.
I've deliberately changed the wording to clarify the concept.

The reasons that earth-leakage devices are fitted to circuits is usually to

protect the cable in the event of fire / over-heating

prevent death in the unlikely event of a person touching a live wire or connector

prevent the metal casing on an appliance becoming live due to a short-circuit fault

That decision is the same irrespective of whatever aberrations are present on the sine wave.

In the case of our installation site, I see value in having the RCD fit, so we will have it. In many other cases, since I understand isolators are typically installed near the heatpump (EST guidance), unless the RCD would remove the need for such isolator, it seems to mostly add to costs without significant value.

But my angle was actually on whether an RCD could be justified solely based on the likely DC components?

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC

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Transparent

(@transparent)

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Joined: 5 years ago

Posts: 3401

12/06/2026 3:28 pm

Posted by: @batpred

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But my angle was actually on whether an RCD could be justified solely based on the likely DC components?

No.

The shape of the waveform has no bearing on the decision to use an RCD.

BTW. I'm still unsure why you're making "DC Component" into a plural.
Could you give an example for me please?

Save energy... recycle electrons!

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Batpred

(@batpred)

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Joined: 2 years ago

Posts: 1159

12/06/2026 9:13 pm

@transparent

i just mean dc in the wire

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC

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