Event Distro Boxes Explained: RCBO vs RCD, Type A vs AC, and Choosing a Distro

What is inside a portable distro, what MCBs, RCDs and RCBOs each do, what Type A, Type AC, B curve and C curve mean on a spec sheet, and how the Elumen8 and PCE boxes we sell compare. Product selection only: no wiring and no design.

~13 min read · Updated October 2026 · By the Stage Supply team

In this guide: The short answer · What’s in a distro · Inputs and outputs · MCB, RCD, RCBO · Type A vs AC · B, C and D curves · Why per-way RCBOs · Reading a spec sheet · Our distros compared · Which box for which job · Popular picks · FAQ

A distro box takes one large supply and splits it into smaller, protected outlets for a stage, a marquee or a site. Two boxes with the same input and the same number of sockets can behave very differently when something goes wrong, and the difference is in the protective devices behind the panel.

This guide explains those devices and how to read a distro spec sheet, so you can choose a box and have a sensible conversation with whoever is designing the system. Connector basics are covered in our event power distribution guide, and rack power strips in 16A and 32A PDUs vs distro, so neither is repeated in depth here.

The short answer

• An MCB protects the cable and equipment against overload and short circuit. An RCD trips on earth leakage, the kind of fault that injures people. An RCBO is both in one device.
• Per-way RCBOs (one RCBO on every outlet) are popular on distros because a fault on one outlet trips that outlet only. A box with one RCD covering every socket loses everything when any one load leaks to earth.
• Type AC vs Type A is about what kind of earth leakage the device can detect. Type AC responds to AC residual current only; Type A also responds to pulsating DC, which electronic equipment can produce.
• B, C and D curve describe how much short-term overcurrent the breaker rides through before tripping instantly. C tolerates more switch-on surge than B.
• Which type and curve a system needs is a design decision for the competent person responsible for it. Your job as a buyer is to make sure the spec sheet says what the box actually has.

What’s in a portable distro

Every distro, from a small 32A box to a 125A or Powerlock unit, has the same three parts:

• An input. A panel inlet or a short trailing lead with a plug, rated for the supply it takes: 32A, 63A or 125A, single-phase or three-phase, or Powerlock single-pole connectors on the largest boxes. Some boxes add a pass-through (also called in/thru) socket so the same feed can continue to the next box.
• Protective devices. MCBs, RCDs, RCBOs or, on the largest boxes, a breaker with an earth leakage relay. Some boxes protect the incoming supply as well as each outlet.
• Outputs. The sockets your equipment plugs into: 13A, 16A and 32A single-phase, 32A or 63A three-phase or Socapex.

The housing matters too. Prolight describes the PCE-built IMST and ISCHL boxes as shock-resistant polyamide and the MERZ boxes as shock-resistant thermoplastic, all rated IP44 (splash-proof). Prolight’s listings describe the Elumen8 boxes as PCE IMST, ISCHL or MERZ boxes, or as built from PCE components in a MERZ enclosure.

Inputs and outputs you will see

Socapex multicores and breakouts are explained in Socapex explained. Powerlock and the CEE-form ratings are in the event power guide.

MCB, RCD and RCBO: what each one does

RCD is the general term. The product standards the IET lists for them call an RCD without integral overcurrent protection an RCCB (BS EN 61008-1) and one with integral overcurrent protection an RCBO (BS EN 61009-1). Hager describes its RCBOs as combining MCB and RCD protection in a single device. The figure you will see on distro RCDs and RCBOs is 30mA, the rated residual operating current.

An MCB on its own does not trip on earth leakage. A box that lists only MCBs on its outlets, such as the PCE IMST 32A to 6 x 16A (9030331), where Prolight lists a Type B MCB on each socket and no RCD, relies on residual current protection being provided elsewhere if the system design calls for it. That is a question for whoever designs the supply, not one to guess at.

Type A vs Type AC (and F and B)

The type of an RCD or RCBO tells you what kind of leakage current it is designed to detect. It has nothing to do with the B, C and D curve, which belongs to the overcurrent part. Hager defines them like this:

Two published points are worth knowing when you see the forum question “RCBOs: A or AC?”. Hager notes that each type is also suitable for the applications of the types before it in the table, so a Type A device is suitable for Type AC applications. And the IET, summarising Regulation 531.3.3 as introduced by BS 7671:2018+A2:2022, says Type AC RCDs shall only be used to serve fixed equipment where it is known that the load current contains no DC components, because DC components can desensitise them. BS 7671 has since been amended again (the current edition is BS 7671:2018+A4:2026, published April 2026), so your designer will work from the current text.

Which type a given system needs remains the designer’s decision. What you can do is check the type is actually stated: several of the boxes in our range say Type A on the manufacturer listing, and several do not state a type at all (see reading a spec sheet).

B, C and D curves

The curve letter describes the magnetic (instantaneous) trip of the overcurrent part of an MCB or RCBO, as a multiple of its rated current. Schneider Electric gives these figures for its Acti 9 breakers; Hager gives the same B and C bands:

In plain terms, a C curve 16A device rides through a bigger switch-on surge than a B curve 16A device before it trips instantly. Sustained overload is handled by the thermal part and is not what the letter describes. Most of the Elumen8 and PCE boxes we sell that state a curve list Type C devices on their outlets; the PCE IMST 32A to 6 x 16A lists Type B MCBs. As with RCD type, whether B or C suits a particular circuit is part of the system design, because the device has to fit the rest of that design.

Why per-way RCBOs are popular on distros

Compare two ways of protecting the same outlets:

• One RCD or RCBO for the whole box, MCBs per outlet. The Elumen8 IMST 32A box (1 x 32A + 4 x 16A) has a Type A 32A 30mA RCBO protecting all outputs and a 16A C curve MCB on each 16A socket. The Elumen8 IMST 6 x 13A with one 32A RCBO puts all six sockets behind one 30mA RCBO. Simpler and lighter, but earth leakage on any outlet switches off every outlet.
• One RCBO per outlet. The Elumen8 ISCHL 32A three-phase box gives each of its six 16A sockets its own 16A 30mA RCBO; the Elumen8 IMST 6 x 13A gives each 13A socket its own 10A 30mA RCBO. A fault on one outlet trips that outlet, and the rest of the stage stays up.
• Something in between. The PCE ISCHL 63A box puts each 32A socket on its own C type 30mA RCBO but wires its six 16A sockets in pairs, each pair on one RCBO.

There is a second reason. Hager explains that equipment with switch-mode power supplies has a standing leakage current to earth in normal use, that this adds up across everything on one RCD, and that an RCD may trip at any value above 50% of its rated residual current. Put a lot of electronic kit behind one 30mA device and the box can trip with nothing actually wrong. Prolight says of its 125A MERZ boxes that individual RCBO protection across all outputs reduces the risk of nuisance tripping. That is the case for per-way RCBOs in one sentence.

The largest boxes add another layer. The Elumen8 MERZ 63A in/thru with variable earth leakage relay and the Powerlock to 125A box list a 4-pole MCB “with variable earth leakage relay” on their three-phase output. Device makers sell time-delayed (S type) residual current devices for the same purpose: Hager describes its S type as ensuring discrimination with downstream 30mA RCDs, limiting nuisance trips. Setting up that kind of upstream protection is squarely a design task.

How to read a distro spec sheet

Manufacturer listings for distros follow a pattern. Read them line by line and check each of these:

• Input: current, voltage, number of pins and whether it is a panel inlet or a trailing lead. “32A 415V 3P+N+E” is a three-phase five-pin input; “32A 230V 2P+E” is single-phase.
• Input protection: whether the incoming supply is protected inside the box (for example “protected by a 4-Pole type C MCB”) or not mentioned.
• Pass-through: present or not, and its rating. It lets the same feed continue to the next box.
• Outputs: number and type of each socket, and for each group, the device behind it.
• Device per output: MCB or RCBO, its current rating, poles, curve (B or C), residual rating (30mA) and RCD type (A or AC). “Each protected by” means per outlet; “protecting all outputs” means one device for the box; “wired in pairs” means two outlets per device.
• Housing, IP rating, size and weight. Useful when the box has to live on a stage edge or travel in a van.

Then notice what is missing. Some real examples from manufacturer listings for boxes we sell:

• The Elumen8 MERZ 63A three-phase RCBO box (A80) lists 3 x 32A and 6 x 16A sockets followed by “each protected by a C type dual pole RCBO”, which does not make clear whether the 32A sockets are included, and it states no residual rating or RCD type. If you need those figures for your paperwork, ask before ordering.
• The Elumen8 MERZ 32A in/thru to 18 x 16A states “type A dual pole RCBO (Type C MCB curve)”, giving both letters and showing they describe different things.
• The Elumen8 ISCHL 63A single-phase box protects its outlets with Type C MCBs behind a single 30mA RCD for the box, so it is a one-RCD box despite its size.
• The PCE ST ANTON 32A three-phase to 3 x 32A lists a 32A C type MCB on each outlet and no RCD.

Do not rely on a shop product title for any of this, ours included. Titles are shortened and sometimes wrong. The manufacturer listing is the reference, and we are happy to send it.

The distros we sell, compared

All figures are from Prolight’s manufacturer listings for the Elumen8 and PCE boxes. 1PH means single-phase, 3PH three-phase. Where a cell says “not stated”, the listing does not give it.

Prolight also lists a Powerlock to 2 x Powerlock splitter, which has no protective devices listed, and black-connector versions of some MERZ boxes, such as the A80BLK, for when the box will be in sight of the audience.

Which box for which job

Start from the supply you will be given, then the outlets you need, then the protection the system designer has asked for. As a guide to where each family fits:

• Small stage or function room on a 32A single-phase socket: an IMST box. Choose per-way RCBOs (4 x 16A RCBO) if you want each position independent, or the lighter one-RCBO layout if the designer is happy with it. For UK-plugged kit, the 13A IMST boxes.
• 32A three-phase from a small generator or a 5-pin socket: the ISCHL 32A three-phase (with pass-through) or the IMST 32A three-phase to 3 x 32A and 3 x 16A.
• 63A three-phase for a main stage or a marquee: a MERZ or the PCE ISCHL 63A. In/thru models let a competent person daisy-chain boxes along one power run.
• Lighting bars on Socapex: the MERZ 63A to Socapex, then breakouts or fixtures.
• Generator or main feeds: 125A MERZ and Powerlock boxes. These sit at the top of a system and are chosen as part of its design.

Three-phase boxes in particular need someone to plan phase balance and supply capacity. Cables between boxes have ratings of their own; see cable drums and reels before feeding a distro from a drum. Distros that go out on hire need to be on a test regime: see PAT testing for event equipment. For a band-sized gig, the stage power checklist is the place to start.

Popular picks from the shop

Frequently asked questions

What is the difference between an RCD and an RCBO?

An RCD trips on residual current, meaning current leaking to earth. An RCBO does that and also trips on overload and short circuit, so it combines an RCD and an MCB in one device for one circuit. RCD is also used as the general term for both.

Should distro RCBOs be Type A or Type AC?

Type AC detects AC residual current only; Type A also detects pulsating DC, which electronic equipment such as LED drivers and switch-mode power supplies can produce. The IET notes that BS 7671, from its 2022 amendment (A2), limits Type AC to fixed equipment known to have no DC components; the current edition is BS 7671:2018+A4:2026. The choice for a particular system belongs to its designer, so check the spec sheet states the type.

B curve or C curve RCBOs for a distro?

B curve devices trip instantly at 3 to 5 times their rating, C curve at 5 to 10 times (Schneider figures), so C rides through larger switch-on surges. Which suits a given system is a design decision. Most of the Elumen8 and PCE boxes we sell that state a curve list Type C.

Why do distros have an RCBO on every socket?

So a fault on one outlet trips only that outlet, and so normal leakage from lots of electronic kit is not added up behind one 30mA device. Hager notes that standing leakage from electronic equipment adds up on a shared RCD, and Prolight says individual RCBO protection on its 125A MERZ boxes reduces the risk of nuisance tripping.

What does in/thru or pass-through mean on a distro?

The box has an output socket of the same rating as its input, so the feed can continue to another box. Whether and how boxes are chained is part of the system design.

Who can set up a distro system?

Design, installation, inspection and testing of temporary event power is work for a competent person, and BS 7671 and BS 7909 apply. Buying the right box is the first step, not the whole job.