Fuse Box, Part P & Earth Loop Testing — Explained
Everything a UK homeowner needs to understand about modern consumer units, the rules that govern them and the testing that proves they're safe.
From "fuse box" to modern consumer unit
The British "fuse box" has evolved through wire fuses → MCBs → dual RCD boards → full RCBO boards. We only fit the modern standard: full RCBO consumer units in steel enclosures with surge protection ready.
Part P in plain English
Part P of the Building Regulations means certain electrical work in your home must be either done by a registered electrician (who self-certifies and notifies) or signed off by Building Control. Notifiable work includes:
- New circuits anywhere in the house
- Replacing a consumer unit / fuse box
- Any work in a bathroom or shower room
- Any work outside (garden lights, outdoor sockets, EV charger)
- Work in a kitchen if it involves new circuits
As a registered Part P scheme member, every notifiable job we do comes with a certificate and Building Control notification — protecting you when you sell the house or claim on insurance.
Earth fault loop impedance testing
A vital test that measures the resistance of the earth-return path on every circuit. A low reading means a fault will trip the breaker in milliseconds; a high reading means the circuit isn't safely protected. We test every circuit on every EICR, rewire and consumer unit upgrade — using a calibrated Megger MFT 1741 for traceable, certified results.
Fuse box change or Part P notification?
Full RCBO upgrades from £650 plus VAT including notification. CHAS approved.
Frequently Asked Questions
What is Part P of the Building Regulations?
Part P is the section of the Building Regulations covering electrical safety in dwellings in England & Wales. Certain work — new circuits, consumer unit replacements, work in special locations like bathrooms — is "notifiable" and must be done by a registered electrician (or notified to Building Control). We are a registered Part P scheme member, so the certificate and notification are handled for you.
How do I change a fuse in a fuse box?
Modern consumer units use MCBs or RCBOs — you simply switch them back on after finding the fault. Old wire-fuse boards use rewireable fuses where you replace the fuse wire to the correct rating (5A lighting, 15A immersion, 30A ring main). If you have a wire-fuse board it is at least 30 years old and we strongly recommend an RCBO upgrade for safety.
What is earth fault loop impedance testing?
Earth fault loop impedance (Zs) measures how quickly a circuit will disconnect during an earth fault. The lower the reading, the faster the breaker trips and the safer the circuit. Every circuit in an EICR or new install gets tested — we use a calibrated Megger MFT for accurate, certified readings.
Are RCBO controlled circuits the same as RCD controlled?
Both detect earth leakage, but an RCBO protects only one circuit while an RCD protects a group. Modern consumer units use one RCBO per circuit so a fault in the kitchen never knocks out the freezer or alarm. We only fit full RCBO boards.
Do I need to replace my old fuse box?
If you have any of: wire fuses, plastic enclosure (pre-2016), no RCD protection, scorching, or it is full and you need more circuits — yes. A full RCBO consumer unit upgrade in Devon starts at £650 plus VAT including certification and Building Control notification.